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Applied Practice in

Reactions and Stoichiometry

AP* Chemistry Series RESOURCE GUIDE

Volume 2

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APPLIED PRACTICE Resource Guide


Teacher Notes and Strategies

A Note for Teachers.............................................................. 5

Teaching Strategies ............................................................... 8

Glossary of Terms............................................................... 15

Student Practices

Multiple-Choice Questions ................................................. 21

Masses and the Mole .......................................................... 22

Chemical Equations and Stoichiometry ................................. 27

Stoichiometry Applications.................................................. 32

Types of Reactions. ............................................................ 37

Laboratory Applications...................................................... 41

Free-Response Questions.................................................... 47

Answer Key and Explanations

Multiple-Choice Answer Key ............................................. 57

Multiple-Choice Answer Explanations............................... 61

Free-Response Answers and Scoring Guides ..................... 71

*AP is a registered trademark of the College Entrance Examination Board, which was not involved in the production of, and does not endorse, this product.

Teacher Notes and Strategies

for

Reactions and Stochiometry

A NOTE FOR TEACHERS

The Applied Practice in AP Chemistry series was designed for use by teachers as an instructional supplement to major units in the AP Chemistry curriculum. This series was also conceived as a resource for teachers in preparing students for the AP Chemistry Exam. As you teach each unit, your students will have the opportunity to practice and to develop those skills required on the exams. Each book in the series includes:

• Teaching notes and strategies • Glossary of terms • 75 multiple-choice questions replicating Section I of the AP Chemistry Exam • Multiple-choice answer keys and answer explanations • 6 free-response questions replicating Section II of the AP Chemistry Exam • Free-response answer keys and scoring guide

We offer a few suggestions and explanations to help you receive the maximum benefit from our materials:

1. Applied Practice booklets do not purport to duplicate exactly an Advanced Placement Examination. However, questions are modeled on those typically encountered on these exams. Thus, students using these materials will become familiar and comfortable with the format, question types, and terminology of Advanced Placement Examinations.

2. Each Applied Practice booklet focuses on one topic within the AP Chemistry

curriculum. These booklets are excellent resources for teachers and their students. Their unique format includes questions designed for use during the initial teaching of the required topics. Other questions are exceptional for the review phase of the course, as students pull the entire year together leading up to the AP Chemistry Exam. The AP exam often will require knowledge in multiple content areas on the same question.

3. You have the option of using the Applied Practice booklets for your own lesson

and test preparation or, if you so choose, students may work through an Applied Practice test booklet on their own as they progress though the course. The students can check their own answers with the answer key and read the answer explanations provided in the teacher edition, conferring with the teacher as needed.

4. The order of topics in the Applied Practice booklets has been organized to follow a logical progression that is similar to the sequence in many of the most widely selected AP chemistry textbooks. You will find that they can easily be adapted to whatever sequence you find most productive at your school.

5. The free-response questions in each topic were created to provide practice questions similar to both those given in part A of the AP Chemistry Exam, which allows use of a calculator, and those given in part B, in which no calculator is allowed. In a few cases, the specific content is best assessed with a combination of both types.

6. Due to the emphasis on laboratory experience in the College Board’s AP Chemistry program, the Applied Practice booklets in AP Chemistry frequently include laboratory-based questions appropriate to the subtopic addressed. A required laboratory-based question does appear on the AP Chemistry Exam. While most Applied Practice booklets in the AP Chemistry series do contain laboratory-based free-response questions, some topics do not lend themselves to the College Board-recommended laboratory experiments. However, each Applied Practice booklet does contain multiple-choice questions related to both laboratory and descriptive chemistry. Only one of the six free-response questions included on the AP Chemistry Exam is laboratory based.

7. Each booklet includes a glossary of terms that applies to the vocabulary of that

particular topic. 8. If the teacher wishes to replicate the conditions under which students will take the

actual AP Chemistry Exam, he or she should understand the following about multiple-choice versus free-response questions when using Applied Practice booklets: When answering multiple-choice questions (AP Exam, Section I) students are not allowed the use of a calculator, and the only reference information available to them is a periodic table (with only symbol, mass number, atomic number) and a small table of abbreviations/symbols used in the questions. When answering free-response questions (AP Exam, Section II), much more information is available to the student. In addition to the periodic table, a table of standard reduction potentials in aqueous solutions and a relatively complete list of equations, constants, and abbreviations/symbols are provided.

COPYRIGHT NOTICE

The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Reproduction of individual worksheets from this booklet, excluding content intended solely for teacher use, is permissible by an individual teacher for use by his or her students in his or her own classroom. Content intended solely for teacher use may not be reproduced, stored in a retrieval system, or transmitted in any way or by any means (electronic, mechanical, photocopying, or otherwise) without prior written permission from Applied Practice. Reproduction of any portion of this booklet for use by more than one teacher or for an entire grade level, school, or school system, is strictly prohibited. By using this booklet, you hereby agree to be bound by these copyright restrictions and acknowledge that by violating these restrictions, you may be liable for copyright infringement and/or subject to criminal prosecution.

TEACHING STRATEGIES

Following are some suggestions for using Applied Practice materials as you work to help your students develop mastery in answering AP-style questions. GENERAL The AP Chemistry Exam will be a challenge for all students. In this section, we have provided strategies for answering the questions themselves as well as some suggestions for teachers to keep in mind when presenting the material. 1. A certain amount of rote memorization is required. We offer the following

suggestions to help students prepare for this:

• Mnemonic devices are excellent tools for memorizing hard facts. For example, in Kings Play Chess On Funny Green Squares, the first letter of each word represents the divisions of the classification system in binomial nomenclature: kingdom, phylum, class, order, family, genus, and species.

• Students are also encouraged to create flashcards; a tried and true method that can help them be successful.

Items that students are required to know for the AP Chemistry Exam include: • names and formulas of polyatomic ions • names and formulas of strong acids and bases, as well as common acids and bases • the metric prefixes from pico to giga • the Greek prefixes in molecular nomenclature: mono, di, tri, tetra, penta, etc. • the solubility rules for ionic compounds • the identity of the seven diatomic elements • the phases of all elements at room temperature (25oC) • the SI quantities and their units • rules for significant figures • rules for assigning oxidation numbers to atoms in compounds

2. Students will see many graphs in both the multiple-choice and free-response

questions. In a few instances, they may be required to produce the graph from given information. Teachers should incorporate interpretation of scientific data written in graphs during instruction both in the classroom and the laboratory. The more graphs that students are required to produce during the year, the easier the questions containing this type of information will be for them on the exam.

MULTIPLE-CHOICE QUESTIONS (MC): 1. The questions in these booklets are valuable for both the initial teaching of the

concepts of chemistry and for preparing students for the style of questions and the level of difficulty encountered on the AP Chemistry Exam. In particular, some subtopics are more fundamental than others. While these subtopics are required in order for students to be able to comprehend more advanced material later on, they would not necessarily appear “as is” in a question on the AP exam. However, we have included multiple-choice questions over this material to support instruction, to provide an expanded test bank of questions, and to familiarize students with this type of assessment.

2. The question most commonly asked by students preparing for the AP exam is: “How

many multiple-choice questions should I answer in Section I?” To answer this question, we will consider what is required for a student to receive a score of “3” (which is labeled as “qualified” by the College Board and considered “passing”; thus, this score is a reasonable goal for most students.) Judging by released past exams for which full data is available, a student typically must score approximately 52–58 points out of the160 possible on Sections I and II combined in order to receive a “3.” Thus, it is reasonable to assume that to pass the AP Chemistry Exam, students need to earn 26–29 points or more from Section I. For each question answered incorrectly, a 0.25 point penalty will be subtracted from the score. Typically, if students are answering more questions correctly than incorrectly, they should continue to increase the number of questions they answer, for it takes four incorrect answers to cancel one correct answer. Most students reach a point at which answering additional questions merely lowers their score. Successful students over the years typically answer the following numbers of questions corresponding to these eventual scores:

• 35–50 questions: “3” • 45–60 questions: “4” • 55–70 questions: “5”

This formula by no means guarantees a particular score; each student is unique in his or her approach to answering questions. This is simply offered as a guideline; an average number of questions a student may choose to answer out of the 75 total questions in an effort to attain a particular achievement level.

3. Another question related to multiple-choice questions on Section I of the AP Chemistry Exam is, “Should I guess?” Because of the penalty for wrong answers (-0.25) and the fact that there is no penalty for questions not answered (just no points earned), it is advisable not to answer questions if the student has little or no knowledge of the information addressed in the question. However, if students can eliminate one or more of the answer choices, it may be a good idea to select an answer, as statistically the odds are then in their favor overall for those questions.

4. A general strategy for multiple-choice questions involves time management. Since the exam limits students’ time to 90 minutes for Section I, they will need to learn to be efficient. While some students simply work more quickly than others, virtually all students find that time is a limiting factor on the AP Chemistry Exam. The ability to read and comprehend quickly is a tremendous advantage. Another skill that will help students overcome the time factor is reading and assessing the level of difficulty of each multiple-choice question quickly. Those questions that students believe they can answer but will take quite a bit of time to analyze should be skipped over initially. Students can mark those questions and return to them later, provided there is enough time. The vast majority of questions answered correctly by students are those for which they can determine the answer almost immediately. Those questions that consume the most time are also those that are most likely to be answered incorrectly.

5. As students approach AP-level question sets, teachers should prepare students to be

satisfied with a lower percentage of correct answers than is typical for them up to this point in their educational experience. Most schools around the country consider 90% an “A” grade, and many AP chemistry students are disappointed when earning test scores in the 70% or 80% range. However, the AP Chemistry Exam is scored in such a way that a successful “passing” score is between 34% and 50%, and a “5”—the highest score possible—is achieved with any point total above approximately 63%. Thus, it will take some time for students to adjust psychologically. Students should understand that a typical passing rate nationwide on the AP Chemistry Exam is approximately 56% of those taking the exam. It is definitely an achievement for a high school student to find success with a “3,” “4,” or “5” on one of the most challenging AP exams. One way to help students cope with a lower percentage of correctly-answered questions is to point out that identifying one’s weakest areas is actually very beneficial when taking the exam. Students who can identify these questions quickly and spend little time on them can accumulate precious minutes to use on other questions, thus raising their score.

6. The percentage scores on AP exams noted above refer to data published by the

College Board on their released questions and will vary from year to year, as individual students and classes of students vary in their abilities from year to year. Also keep in mind is that when students are being assessed on tests in class during the school year, they are often assessed on a concentrated, narrow amount of information – helpful for formative assessment and preparation, but not necessarily characteristic of a final AP exam administered after course has been completed. While students may achieve a higher degree of success on AP-level multiple-choice questions when they are tested immediately at the end of a unit; teachers are encouraged to factor in the level of difficulty and drastically reduced raw score that is regarded as highly successful by the College Board when assessing the students’ efforts.

7. AP chemistry students typically do struggle with AP-level questions early in the course. AP-level questions require a broad background in the overall chemistry curriculum, and are also more difficult than those most students have encountered up to this point. It is advisable to provide answers to practice questions for students, especially early in the school year. They will find it helpful to read the question, work out an answer, and then get immediate feedback. Providing answer keys allows students to work backward from the answer and develops their understanding of how to attack multiple-choice questions at this level of difficulty. The answer explanations provided in the Applied Practice booklets are particularly useful in this regard. In addition to allowing some access to answer keys (unless, of course, you will be using these questions for assessment), teachers should plan to include time in class for guided practice of these questions, working them out with the students. Collaboration among students in small learning groups is also a useful technique. Chemistry is much easier to learn when students are able to talk about the content and reason out answers together.

8. A simple but useful tip for students is to underline, or in some way highlight, key

terms in the questions. Many times, a student will choose an incorrect answer because he or she simply misread the question.

FREE-RESPONSE QUESTIONS (FRQs): 1. One of the most common reasons for producing an unsatisfactory response to a free-

response question on an AP Chemistry Exam is that the student did not answer the specific question that was asked. Often, students write information that is factually correct but does not apply to the question, or their response relates to information in the question without actually answering the question. Free-response questions often present a great deal of information when only small bits of that information are actually needed to answer each of many different parts of the question. Students should read the entire question before they begin to write. They should underline, or in some way highlight, exactly what is being asked in each part of the question. After writing their response, students should proofread their work to be sure they answered the question specifically. Often, students will look back over their answers when their papers are returned and realize their answers make little sense, often responding, “But I meant to say ____.” They will likely experience greater success when they take time to read over their answers to verify they actually wrote what they intended to say.

2. Students should be specific in their answers and should understand the difference

between a definition and information that justifies or explains their answer. In most cases, students should use complete sentences. A few questions have parts that merely require a list of items, and others ask for additions to diagrams, but for the most part, students will need to write coherently. A one-word answer is not sufficient to earn credit.

3. Students should be sure they understand the difference between an answer that

justifies their point and one that merely cites a correlation. A common example on past tests is a question about what determines the relative strength of London dispersion forces between molecules. Larger-sized atoms and molecules have more electrons and are more easily “ionizable”; thus, they are more likely to form dispersion forces. As atoms and molecules get larger, their nuclei get heavier, and thus the atomic and molecular weights increase along with dispersion forces. However, while increasing mass correlates to increasing strength of attraction between molecules, it does not explain or justify the increase.

An analogy illustrating this concept would be a graph of children’s mass with their level of intelligence. As they age, their brains develop at the same time their bodies grow and become heavier. Thus, older children are both smarter and heavier than younger children. But it would be incorrect to say that the children are becoming smarter because they are becoming heavier.

4. When teaching concepts in chemistry, it is tempting to make atoms, ions, and

molecules anthropomorphic. Humans tend to more easily understand concepts that are explained in a way that is relevant to the way we think and behave. Teachers must be careful not to instruct the students about what the atoms “want” or “need.” Often,

students will write answers about an atom that is “happy” because it got the electron it “needed.” Even though students may have come to an appropriate level of comprehension on this concept, they will not receive credit when citing the emotions of a piece of matter on the AP Chemistry Exam.

5. Students should write out words fully, unless the abbreviations are universally

accepted. While their teachers may have come to understand a particular student’s shorthand, students are likely to lose out when the AP exams are graded.

6. Students should develop the habit of avoiding pronouns in their written answers, such

as beginning their first sentence with “it.” In many questions, there are dozens of things to which “it” could refer, and students will receive no credit for their answer if they use vague pronouns.

7. Using the free-response questions in the Applied Practice books is a good way to

develop students’ writing skills. Answers, explanations, and scoring guides for each of the six questions in an Applied Practice booklet are provided. The more writing students do, the better they will become at answering the questions in Section II of the AP exam. Although it takes time to grade students’ writing, the time will be well worth it in terms of their improvement and eventual success. Free-response questions are of three general types: large calculation questions that require good problem solving skills, equation writing, and “essay” types that require good writing skills. The first type of question will be seen on Section A of Part II of the exam, and the last two types will be found on Section B of Part II.

8. When answering free-response questions that require calculations (section II, part A,

questions 1, 2, and 3), students should develop these good habits from the beginning of the year:

Always list any mathematical formula to be used (i.e. PV=nRT) at the beginning of

the question. Never put numerical information in an equation without first writing that equation down first. The vast majority of all mathematical formulas to be used are listed in the reference sheets provided on the AP Chemistry Exam in section II. Some knowledge of mathematical formulas will be required on section I (multiple- choice), so it is good for students to know them and not always rely on the reference sheets.

While students’ work must be shown, typically much less work is required than that

required by most high school math teachers. If the student lists the mathematical formula, substitutes the correct numerical values from the question into the proper location, and clearly marks the answer, the student does not need to write out each of the various algebraic steps. However, there is no disadvantage in scoring if students are more confident showing their work. They simply need to be coached in the amount of time to use on each calculation question. Some students will need to write fewer steps if time becomes a factor overall.

Once the students calculate and write a final answer, they should do the following each time:

• Double check the number of significant figures in the answer. They must be

within one significant figure up or down from the correct number. • Double check the calculations. • Make sure they have labeled their answer as to which quantity it is (do not leave

answer as “x=0.050”). A label helps ensure they are answering the question correctly.

• Always include units (except for a few that do not use units, such as specific gravity and the equilibrium constant, Keq, etc).

• Box in the final answer. This not only helps the grader, but it provides an easy way for the students to check their work to ensure they have finished each part of each question. Also, answers from previous sections are often used on subsequent parts of the question. It is easier to find this information if it is boxed.

GLOSSARY OF TERMS

anhydrous—without water; a crystal that has had water molecules driven off through heating; has a similar formula to a hydrate; ex: hydrated crystal CuSO4∑5H2O, anhydrous crystal CuSO4 anion—a negatively charged ion, an atom or molecule that has gained one or more electrons aqueous—dissolved in water, in a water solution atom—an extremely small particle of matter, the smallest particle of an element which retains the properties of that element atomic mass—(atomic weight) the average atomic mass for the naturally occurring isotopes of an element balanced equation—chemical equation representing conservation of mass, where the same number of atoms of each element appear on both sides (before and after the arrow) catalyst—a substance that increases the rate of a chemical reaction without being consumed in the reaction cation—a positively charged ion, an atom or molecule that has lost one or more electrons coefficient—number written before a formula in a balanced equation to show the relative number of moles of that substance in the reaction combustion—burning, a type of reaction where a substance reacts with molecular oxygen, forming oxides compound—a substance composed of two or more elements that are chemically combined, in fixed ratios with its own set of properties different from the elements’ concentration—a general term concerning the relative quantity of material (solute) dissolved into a solution decomposition—a type of reaction in which a single compound reacts forming two or more substances, often requiring heating displacement—(single displacement) a type of reaction in which an element reacts with a compound, displacing an element within the compound (takes its place) empirical formula—(simplest formula) the formula of a substance written with the set of smallest whole number subscripts

equation—a set of chemical formulas written to represent the substances that undergo chemical changes in a reaction, written as: reactants products formula—a set of chemical symbols possibly with subscripts written to represent the relative (or exact if molecular) number of atoms of each element in a compound gravimetric analysis—a type of quantitative analysis in which the amount of a particular substance within a material is determined by converting that substance into a product that can be completely isolated from others and weighed hydrate—a compound that contains water molecules weakly bound in a crystal hygroscopic—the characteristic of naturally absorbing moisture from the air limiting reactant—(limiting reagent) the reacting substance that is entirely consumed, thus running out and causing the reaction to stop mass percent—(mass percentage) mass of one element in a compound relative to 100 grams of that compound molar mass—the mass of one mole of a substance, in grams, equal to the sum of the atomic masses of the elements in the formula for that substance molecule—the smallest physical unit of a substance that can exist independently, consisting of one or more atoms held together by covalent bonds net ionic equation—a chemical equation in which all substances (species) are written as they exist (solid, aqueous, etc) and substances that do not change during the reaction (spectator ions) are not included (canceled) oxidation number—(oxidation state) either the actual charge on atom in a substance or the hypothetical charge assigned by simple rules oxidized—(oxidation) the action of having lost electrons in a reduction-oxidation reaction (redox reaction), resulting in an increase in oxidation number oxidizing agent—the substance in a redox reaction that oxidizes another substance; it is the substance that is reduced in the reaction percent composition—the mass percentages of each element in a compound percentage yield—the actual yield (from experimental data) as a percentage of the calculated theoretical yield

physical property—a characteristic of a substance that can be determined without changing its chemical identity (ex: color, density, hardness, etc) precipitation—the formation of an insoluble solid compound during a chemical reaction in a solution product—a substance that results from a chemical reaction; written after the arrow reaction—the process of substances going through chemical changes redox—(reduction-oxidation) a type of reaction in which electrons are exchanged and certain elements change oxidation number reduced—the process of gaining electrons during a redox reaction; the oxidation number decreases reducing agent—the substance that causes reduction in another substance; it gets oxidized salt—an ionic compound, typically the product of a neutralization reaction between and acid and a base species—any specific chemical substance that is part of a chemical system or chemical reaction stoichiometry—the use of mole ratios of substances in a balanced chemical equation; calculations with the relative coefficients of two substances to determine reacting quantities symbol—(chemical symbol) one capital letter or one capital letter and one lower case letter* that represent a chemical element; often representing one atom of that element *(elements newly discovered or referred to theoretically that have not officially been given a name will have three letters) synthesis—(combination) a type of reaction in which two or more simpler substances are chemically reacted to form a compound water of crystallization—the molecules of water that are weakly attached in a hydrated crystal

Student Practices

for


Multiple-Choice

Questions

Masses and the Mole

Questions 1-3 refer to the following series of answer choices. They may be used once, more than once, or not at all. (A) 1 mole (B) 2 moles (C) 3 moles (D) 5 moles (E) 8 moles 1. The number of moles of NH3 molecules in 17.0 g of NH3 2. The number of moles in 3.011 x 1024 atoms of carbon atoms 3. The number of moles of sulfur atoms in 6.022 x 1023 S8 molecules. 4. Which of the following statements is true? I. The molar mass of CaCO3 is 100.091 g mol-1. II. 50 g of CaCO3 contains 9.033 x 1023 oxygen atoms. III. A 200.182 g sample of CaCO3 contains 2 moles of CaCO3. (A) I only (B) II only (C) III only (D) I and III only (E) I, II, and III

5. Which of the following represents the correct method for converting 11.0 g of copper metal to the equivalent number of copper atoms?

(A) 2311.0g of Cu atoms 1 mole of Cu atoms 6.022 x 10 Cu atoms

63.55g 1 mole of Cu atoms⎛ ⎞⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠⎝ ⎠

(B) 11.0g of Cu atoms 1 mole of Cu atoms63.55g

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

(C) 23

11.0g of Cu atoms 1 mole of Cu atoms 63.55 g of Cu63.55g 6.022 x 10 moles of Cu atoms

⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠⎝ ⎠

(D) 2311.0g of Cu atoms 63.55g 6.022 x 10 Cu atoms

1 mole of Cu atoms 1 mole of Cu atoms⎛ ⎞⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠⎝ ⎠

(E) 23

11.0g of Cu atoms 1 mole of Cu atoms 1 mole of Cu atoms63.55g 6.022 x 10 Cu atoms

⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠⎝ ⎠

6. Which of the following contain approximately 3.011 x 1023 particles? (A) 32.0 g of oxygen molecules (B) 16.0 g of oxygen molecules (C) 16.0 g of oxygen atoms (D) 4.0 g of helium atoms (E) 23.0 g of sodium ions 7. Which of the following contain(s) the same number of atoms as 48.6 g of magnesium? (A) 1 mole of calcium atoms (B) 56.028 g of nitrogen molecules (C) 48.6 g of calcium atoms (D) 4 moles of magnesium atoms (E) 1.204 x 1024 atoms of carbon

8. 12.011 g of carbon atoms contain the same number of atoms as which of the following? I. 6.022 x 1023 atoms II. 22.99 g of Na atoms III. 6.022 x 1023 molecules of Cl2 gas (A) I only (B) II only (C) III only (D) I and II only (E) II and III only 9. Which of the following statements is INCORRECT? I. 8.0 g of oxygen atoms contains the same number of moles of particles as 16.0 g of O2

molecules. II. 120.11 g of carbon atoms contains 6.022 x 1023 atoms. III. The conversion shown below correctly converts 1 mole of Be atoms to the equivalent

number of grams of Be. 23

23

1 mole of Be atoms 6.022 x 10 Be atoms 9.012 g of Be atoms1 mole of Be atoms 6.022 x 10 Be atoms

⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠⎝ ⎠

(A) I only (B) II only (C) III only (D) I and II only (E) II and III only 10. Which of the following choice contains the least number of ATOMS? (A) 1 mole of F2 molecules (B) 38.0 g of F2 molecules (C) 19.0 g of F2 molecules (D) 38.0 g of F atoms (E) 2 mole of F atoms

11. All of the following quantities have a mass of 79.9 g, EXCEPT (A) 0.50 moles of Br2 molecules (B) 1 mole of bromine atoms (C) 6.022 x 1023 bromine atoms (D) 3.011 x 1023 Br2 molecules (E) 0.5 moles of bromine atoms 12. Which of the following choices is the mass of 0.25 moles of sodium carbonate, Na2CO3? (A) 105.991 g (B) 52.996 g (C) 26.498 g (D) 0.25 g (E) 1.5055 x 1023 g 13. Which of the following is always true of a correctly balanced chemical reaction? I. The number of moles is the same on each side of the equation. II. Moles are conserved during the reaction. III. The total of the average atomic masses on each side of the equation must be the same.

(A) I only (B) II only (C) III only (D) I and II only (E) I and III only

14. Which of the following statements is true?

I. 34.1 g of NH3 is the equivalent of 6.022 x 1023 molecules. II. 6.022 x 1023 molecules of NH3 contains three times as many H atoms III. 6.022 x 1023 molecules of NH3 contains 28.0 g of nitrogen


15. If 1.5505 x 1023 atoms of an unknown element have a mass of 12 g, what is the molar mass

of the element?

(A) 3.0 g (B) 12 g (C) 24 g (D) 48 g (E) cannot be calculated from the information provided

Chemical Equations and Stoichiometry Questions 16-18 refer to the following series of answer choices. They may be used once, more than once, or not at all.

(A) 2Na+ + O2- Na2O (B) 2Na+ + O2

2- Na2O2 (C) 2Na + O2 Na2O2 (D) 4Na + O2 2Na2O (E) 2Na+ + O2 Na2O2

16. Which equation is NOT balanced in mass and charge? 17. Which equation shows the molar relationship between the oxygen-containing species on the

left, and product on the right, to be 1:2? 18. Which equation shows the LEAST number of atoms being produced? 19. When the following chemical equation is balanced using the lowest possible whole numbers,

what is the sum of the stoichiometric coefficients? SiCl4 + H2O Si(OH)4 + HCl

(A) 4 (B) 6 (C) 8 (D) 10 (E) 20

20. When the following chemical equation is balanced using the lowest possible whole numbers,

what is the sum of the stoichiometric coefficients?

H3PO4 + Mg(OH)2 Mg3(PO4)2 + H2O

(A) 2 (B) 3 (C) 4 (D) 6 (E) 12

21. When balancing the equation for the combustion of pentane (C5H12) with oxygen gas to

produce carbon dioxide and water using the lowest possible whole numbers, what is the stoichiometric coefficient for water?

(A) 1 (B) 5 (C) 6 (D) 8 (E) 10

22. Consider the equation below. When correctly balanced, how many moles of NO would be

formed from 3 moles of NCl3? NCl3 + H2O HCl + NO + NO2

(A) 0.5 (B) 1 (C) 1.5 (D) 3 (E) 6

23. When balanced correctly, which of the following statements about the following chemical

reaction is true? H2Se + O2 H2O + SeO2

I. One mole of H2Se will yield two moles of H2O. II. Two moles of H2Se will yield two moles of H2O. III. Four moles of H2Se will need six moles of oxygen to react completely.

(A) I only (B) II only (C) III only (D) II and III only (E) I, II, and III only

24. When the equation below is correctly balanced and all coefficients are reduced to their lowest whole numbers, the coefficient for Fe2+

(aq) is MnO4

-(aq) + H+

(aq) + Fe2+(aq) Fe3+

(aq) + Mn2+(aq) + H2O(l)

(A) 1 (B) 2 (C) 4 (D) 5 (E) 8

25. When the equation below has been balanced, how many moles of Sn2+

(aq) are required to react completely with 3 moles of Cr2O7

2-(aq)?

Cr2O7

2-(aq) + Sn2+

(aq) + H+(aq) Sn4+

(aq) + Cr3+(aq) + H2O(l)

(A) 1 (B) 2 (C) 3 (D) 4 (E) 9

26. Using the equation below, determine which of the statements that follow is true. BrO3

- + 5Br- + 6H+ 3Br2 + 3H2O I. The moles of H+ required to react with 3 moles of BrO3

- can be calculated thus;

- +

3-

3

3 moles of BrO 6 moles of H1 mole of BrO⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

II. The moles of Br2 and H2O produced will always be the same III. The equation is balanced in terms of both charge and mass.

(A) I only (B) III only (C) I and II only (D) II and III only (E) I, II, and III

27. When the equation below is balanced, which of the following statements is true? S2O3

2-(aq) + I2(aq) S4O6

2-(aq) + I-(aq)

I. When 1 mole of I2 is reacted with excess S2O3

2-(aq), 1 mole of S4O6

2-(aq) is produced.

II. The amount of I-(aq) produced is twice the amount of S4O6

2-(aq).

III. Four sulfur atoms appear on each side of the balanced equation.

(A) I only (B) III only (C) I and II only (D) II and III only (E) I, II, and III

28. Assuming that coefficients that are fractions ARE permitted, which of the following

represents a set of coefficients that will correctly balance the following equation? H2S + O2 SO2 + H2O I. 2, 3, 2, 2 II. 1.5, 2.5, 3, 2 III. 1, 1.5, 1, 1


29. Which of the following equations correctly shows the balanced equation for the formation of

carbon dioxide and water from the combustion of methane?

(A) CH4 + O2 CO + H2O (B) CH4 + 2O2 CO2 + 2H2O (C) 2CH4 + O2 2CO + 2H2O (D) CH4 + O2 CO2 + H2O (E) 2C2H6 + 7O2 4CO2 + 6H2O

30. Which of the following sets of stoichiometric numbers is correct for balancing the chemical

equation below? Fe2O3 + NH3 Fe3O4 + N2 + H2O (A) 9, 2, 6, 1, 3

(B) 3, 2, 2, 1, 1 (C) 1, 2, 6, 1, 3 (D) 9, 2, 6, 2, 3 (E) 5, 1, 3, 1, 2

Stoichiometry Applications Questions 31-33 refer to the following series of answer choices. They may be used once, more than once, or not at all.

(A) CH (B) CH2 (C) CH3 (D) CH4 (E) C2H6

31. The empirical formula for butene, which has the molecular formula C4H8 32. The empirical formula of a hydrocarbon which is 80% C by mass 33. The formula in which there is 25% H by mass 34. During the synthesis of a solid organic compound the following data are either collected or

calculated:

Mass of dry filter paper = 2.456 g Mass of dry filter paper + dry organic product = 4.345 g Expected yield of dry organic product = 2.976 g

Which shows the correct setup for the calculation of the percentage yield?

(A) ( )4.345 g - 2.456 g x 100

2.976 g⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

(B) ( )2.456 g - 4.345 g x 100

2.976 g⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

(C) ( )4.345 g - 2.976 g x 100

2.456 g⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

(D) ( )2.976 g - 2.456 g x 100

4.345 g⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

(E) ( )2.456 g - 2.976 g x 100

4.345 g⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

35. What is the mass percent of calcium chloride?

(A) 25% Ca and 75% Cl (B) 50% Ca and 50% Cl (C) 40% Ca and 71% Cl (D) 36% Ca and 64% Cl (E) 66% Ca and 33% Cl

36. Which of the following sets includes compounds that have an identical empirical formula?

(A) C2H6, C4H8, C6H6 (B) C2H6, C3H8, C4H8 (C) CH4, C2H6, C3H8 (D) C2H2, C4H4, C6H6 (E) C2H2, C2H4, C2H6

37. Which of the following represents an empirical formula?

I. C2H2 II. N2F4 III. BeCl2 IV. C6H12O6

(A) III only (B) I and II only (C) II and III only (D) I, II, and III only (E) I, II, III, and IV

38. Hydrogen and oxygen can react together to produce H2O according to the equation below: 2H2 + O2 2H2O If 4.0 g of H2 are exploded with 16.0 g of O2, which of the following will be present at the

end of the reaction? I. O2 II. H2 III. H2O

(A) I only (B) I and II only (C) I and III only (D) II and III only (E) I, II, and III

39. A reaction between 246 g of nitrobenzene (C6H5NO2) and an excess of CH3Cl in the

presence of an AlCl3 catalyst yields 54.8 g of a compound with the formula, C7H7NO2. According to the equation below, what is the percentage yield?

C6H5NO2 + CH3Cl →catalyst AlCl3

C7H7NO2 + HCl

(A) 10.0% (B) 20.0% (C) 22.3% (D) 54.8% (E) 100%

Questions 40 and 41 concern the compound calcium carbonate and its reaction with hydrochloric acid which is summarized in the equation below. CaCO3 + 2HCl CaCl2 + CO2 + H2O 40. Which of the following represents the mass percent composition of the elements Ca, C and O

in CaCO3?

41. If 50.0 g of CaCO3 is reacted with an excess of hydrochloric acid and there is a 50% yield, how many grams of carbon dioxide will be formed?

(A) 11.0 g (B) 17.6 g (C) 22.0 g (D) 35.2 g (E) 44.0 g

42. In organic chemistry, the reaction below is used in the identification of compounds. If 6

moles of CH3COCH3 are brought together with 20 moles of NaOH and 24 moles of I2 in a reaction vessel, which species is limiting?

CH3COCH3 + 4NaOH + 3I2 CHI3 + 3NaI + NaCH3CO2 + 3H2O

(A) CH3COCH3 (B) NaOH (C) I2 (D) CHI3 (E) cannot be calculated from the data given


I. NO is 50% N and 50% O by mass. II. N2O4 has an empirical formula that is the same as its molecular formula. III. N2O5 and NO2 have the same empirical formula but different %’s by mass

(A) I only (B) II only (C) III only (D) I and II only (E) None of the statements are true.

44. In which of the following is the mass ratio of S to Cl the same?

(A) SCl and SCl2 (B) SCl3 and SCl4 (C) S2Cl3 and S2Cl4 (D) SCl2 and S3Cl6 (E) SCl and SCl6


I. Because of the efficiency of modern industrial processes, many chemical reactions can be expected to produce yields of 100% or greater.

II. A correct method of calculating % yield is: 100% x Yield lTheoretica

Yield Actual⎟⎠⎞

⎜⎝⎛ .

III. Actual yields of any products are dependent upon the limiting reactant.

(A) I only (B) III only (C) I and II only (D) II and III only (E) None of the statements are true.

Types of Reactions Questions 46-48 refer to the following series of answer choices. They may be used once, more than once, or not at all.

(A) Precipitation (B) Single replacement (C) Synthesis (D) Decomposition (E) Halogen displacement

46. The type of reaction that occurs when AgNO3(aq) is added to NaCl(aq) 47. The type of reaction that occurs when magnesium carbonate is heated strongly 48. The type of reaction that occurs when potassium metal is added to cold water 49. Which of the following statements about the reaction between a solution of copper(II) sulfate

and iron metal is true?

I. It can be classified as a displacement reaction. II. It can be classified as a redox reaction. III. The products would include a sulfate of iron. IV. The products would include solid copper metal.

(A) I only (B) II only (C) I, II, and III only (D) II, III, and IV only (E) I, II, III, and IV

50. Which of the following actions would lead to a reaction that could be classified as a precipitation?

I. AgNO3(aq) is added to NaBr(aq) II. Pb(NO3)2(aq) is added to NaI(aq) III. Zn(s) is added to HCl(aq)

(A) I only (B) II only (C) I and II only (D) II and III only (E) I, II, and III

51. Which of the following statements about the reaction between a solution of nitric acid and

zinc metal is true?

I. A gas is produced. II. A precipitate is produced. III. In the net ionic equation, one of the products would be Zn2+

(aq).

(A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III

52. Which of the following is ALWAYS true of a balanced, net ionic, redox reaction?

I. The species that loses electrons is the reducing agent. II. There is always a loss of electrons by one species, but not always a gain of electrons by

another. III. The atoms and charges must balance. IV. Species that lose electrons are oxidized.

(A) I only (B) I and II only (C) I, III, and IV only (D) II, III, and IV only (E) I, II, III, and IV

53. In acid solution, the MnO4-(aq) ion can act as an oxidizing agent. Which choice describes the

change that the permanganate ion undergoes?

(A) It gains electrons and Mn2+(aq) is a product

(B) It gains electrons and Mn(s) is a product (C) It loses electrons and Mn8+

(aq) is a product (D) It loses electrons and Mn2+

(aq) is a product (E) None of the above

54. On heating a typical group 2 carbonate, which of the following would be an expected set of

products?

(A) the corresponding oxide and CO32-

(B) the corresponding oxide and CO (C) the corresponding oxide and CO2 (D) the metal and CO2 (E) the metal and CO

55. The reaction resulting from the direct heating of solid sulfur in a stream of oxygen gas could

be classified as

(A) decomposition (B) single replacement (C) synthesis (D) double replacement (E) No reaction occurs.

56. When a solution of copper(II) sulfate is added to a solution of sodium hydroxide, the

products include

(A) copper metal and sulfur (B) solid copper(II) hydroxide and a solution that contains sodium and sulfate ions (C) solid sodium sulfate and a solution that contains copper(II) and hydroxide ions (D) sodium metal and water (E) No reaction occurs.

57. Consider the balanced (using lowest possible integers), net ionic equation between sodium phosphate solution and magnesium chloride solution. Which of the following appears on the product side of the equation?

(A) Na+

(aq), Cl-(aq), Mg2+

(aq), PO43-

(aq) (B) NaCl(s), Mg3(PO4)2(s) (C) Mg3(PO4)2(s) (D) Mg2(PO4)3(s) (E) NaCl(s), Mg2+

(aq), PO43-

(aq) 58. Which of the following reactions could be classified as a decomposition reaction?

(A) the strong heating of solid calcium carbonate in the absence of oxygen (B) the addition of hydrochloric acid to sodium hydroxide in a titration (C) the reaction of potassium manganate(VII) with a solution containing iron(II) ions (D) the addition of zinc metal to sulfuric acid (E) the reaction of solid sodium in a stream of pure chlorine gas

59. Consider the balanced (using lowest possible integers), net ionic equation between lithium

metal and nitrogen gas. Which of the following appears on the product side of the equation?

(A) Li3N3(s) (B) Li+

(aq) (C) N3-

(aq) (D) N2(g) (E) Li3N(s)

60. What would be an expected observation when a solution of any group 1 chloride is added to

a solution containing potassium hydroxide ions?

(A) A precipitate would form. (B) No reaction would occur. (C) Chlorine gas would be produced. (D) The group 1 metal would be a product. (E) Hydroxide ions would be reduced to water.

Laboratory Applications Questions 61-63 refer to the following series of answer choices. They may be used once, more than once, or not at all.

(A) Anhydrous salt (B) Hydrated salt (C) Water of crystallization (D) Constant mass (E) Hygroscopic

61. The term that describes the action of a salt as it absorbs water from the atmosphere 62. The term that describes the residue after completely heating a salt to remove the water of

crystallization present in the salt 63. The term that is used to ensure that all of the water has been driven off during heating in an

experiment to determine the water of crystallization present in a hydrated salt 64. The formula of a hydrated salt is to be determined in the laboratory. It is known to have the

formula MgSO4.XH2O. After heating a 24.6 g sample of the hydrated salt to a constant mass, a residue is left that has a mass of 12.0 g. What is the value of X in the formula?

(A) 2 (B) 3 (C) 5 (D) 7 (E) 9

65. A student performed an experiment to determine the value for X in the formula of the hydrated salt, CuSO4.XH2O. A 4.50 g sample of the hydrated salt was heated strongly, then cooled, and its mass was found to be 4.18 g. Which sequence represents the correct order of procedures for completing the experiment?

(A) Subtract 4.18 g from 4.50 g to obtain the mass of water driven off and convert to moles;

convert 4.18 g of anhydrous salt to moles; compare the moles of the anhydrous salt and water to determine the value of X.

(B) Reheat the sample once and then carry out the procedure in (A) with the new mass rather than 4.18 g.

(C) Reheat the sample to a constant mass and then carry out the procedure in (A) with the new mass rather than 4.18 g.

(D) Repeat the experiment with a different starting mass, average the values from the two experiments and then repeat the procedure in (A) with the average masses rather than 4.50 g and 4.18 g.

(E) None of the above procedures will lead to a completed experiment. 66. In an experiment designed to find the concentration of chloride ions present in a sample of

pond water, a solution of silver nitrate was added to the water in order to precipitate the chloride ions as AgCl(s). Which of the following could lead to an incorrect answer being calculated?

I. Failure to realize that the pond water also contained bromide ions II. After filtration, failure to dry the filtrate before weighing it III. Failure to realize that the pond water also contained potassium ions

(A) I only (B) II only (C) I and II only (D) II and III only (E) I, II, and III

67. In an experiment to find the atomic mass of an element T (not its chemical symbol), the analysis of 1.0 mole of each of four different compounds that contains T produced the following data;

A possible atomic mass of T in g mol-1, is

(A) 24.0 (B) 30.0 (C) 31.0 (D) 62.0 (E) 93.0

68. In an experiment to determine the atomic mass of an unknown element J (not its chemical

symbol), it was found that J forms a compound with chlorine that has the formula JO2. The compound contains 40% O by mass. What is the atomic mass of J?

(A) 20 g mol-1

(B) 40 g mol-1 (C) 48 g mol-1 (D) 60 g mol-1 (E) 72 g mol-1

69. An experiment is carried out in order to determine the empirical formula of an ionic metal

oxide. A 0.01 mole sample of metal X, (not its chemical symbol), is heated strongly in a crucible until it has completely reacted with oxygen present in the air. Use the data below to determine the formula of the oxide produced in the experiment.

Mass of crucible + metal = 30.123 g Mass of crucible + contents after heating = 30.282 g

(A) XO (B) XO2 (C) X2O (D) X2O3 (E) X3O4

70. Which of the following aqueous solutions could be used in a gravimetric analysis experiment

that is to determine the mass percent of sulfate ions in an unknown solution?

I. BaCl2 II. NaCl III. MgCl2

(A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III only

71. During an experiment designed to determine the water of crystallization of a hydrated salt, a

sample of the salt is strongly heated. Unknown to the student, some of the salt splatters out of the evaporating dish during the heating. What will be the effect of the loss of salt on the student’s results?

(A) The mass of the salt before heating will be too large. (B) The residue after heating will be recorded as being smaller than the correct value and as

a result the percentage of water calculated will be too large. (C) The residue after heating will be recorded as being larger than the correct value and as a

result the percentage of water calculated will be too large. (D) The residue after heating will be recorded as being larger than the correct value and as a

result the percentage of water calculated will be too small. (E) The residue after heating will be recorded as being smaller than the correct value and as

a result the percentage of water calculated will be too small. 72. Which of the ANIONS in the following aqueous solutions could be quantitatively analyzed

in a gravimetric analysis, by the addition of a solution of AgNO3(s)?

I. NaBr II. NaCl III. NaNO3


73. A 1.00 mole sample of iron metal is heated in a stream of chlorine gas for an extended period of time. It is found that the iron reacts with exactly 71.0 g of Cl2 gas in that time. What is the empirical formula of the compound formed?

(A) FeCl (B) FeCl2 (C) FeCl3 (D) Fe2Cl2 (E) Fe3Cl4

74. Which of the following hydrates would be expected to lose the largest percentage of its mass

when one mole is heated to a constant mass?

(A) MgSO4.7H2O (B) CuSO4.5H2O (C) SnCl2.2H2O (D) CoCl2.6H2O (E) FeCl2.6H2O

75. The following formulas are determined by experiment. Which of the following sets lists the

formula in order of INCREASING mass percentages of oxygen?

I. MgO, Na2O, Na2O2 II. Na2O, Na2O2, BaO III. Na2O, MgO, Al2O3


Free-Response

Questions

1. An experiment is conducted to determine the molar reacting ratio in the reaction between AgNO3(aq) and an aqueous unknown, group 1 phosphate. The method of continuous variation is used. In continuous variation, different amount of reactants are brought together and some physical property resulting from the reaction is measured. In this case, varying volumes (in each case adding up to a total volume of 40.0 mL) of 0.25M solutions are reacted together, a precipitate is formed and its mass is measured.

(a) Data are collected and plotted on the graph in figure 1 below. Add lines to the graph that

will allow the determination of the reacting ratio.

(b) What are the optimum volumes of the two solutions, i.e. the volumes that produce the

greatest amount of product? (c) Write and balance a FULL chemical equation for the reaction, using the symbol X for

the cation associated with the phosphate ion in the unknown solution. (d) Re-write the equation in (c), this time writing the net ionic equation. (e) Moving left to right on the graph, which reactant is limiting AFTER the maximum mass

of precipitate has been formed? Explain your answer. (f) Continuous variation can be used in other reactions where precipitates are not formed.

Suggest another physical property that could be monitored to determine an optimum ratio of reactants

2. For each of the following three statements write the balanced net ionic equation (show significantly ionized species as separate ions and cancel out spectators) and answer the question that follows. There is no need to include phase labels. In every case, a reaction does occur. One point may be awarded for reactants (no partial credit given) up to two points may be awarded for products (partial credit given), one point for balancing and one point for the follow-up question.

(a) (i) A sample of 1-butanol is burned in an excess of oxygen gas.

(ii) Name an isomer of 1-butanol. _________________________________________________________________ (b) (i) Solutions of potassium chromate and lead(II) nitrate are mixed in a test tube

(ii) What color will sodium ions impart to a flame test? _________________________________________________________________ (c) (i) Liquid bromine is added drop wise to a solution that contains lithium iodide.

(ii) How can this reaction be classified?

__________________________________________________________________

3. For each of the following three statements, write the balanced net ionic equation (show significantly ionized species as separate ions and cancel out spectators) and answer the question that follows. There is no need to include phase labels. In every case, a reaction does occur. One point may be awarded for reactants (no partial credit given) up to two points may be awarded for products (partial credit given), one point for balancing and one point for the follow-up question.

(a) (i) Solid cesium oxide is added to water.

(ii) If the resultant solution were tested with red litmus paper, what would one expect to

observe?

__________________________________________________________________ (b) (i) A solution of potassium iodide is added to an acidified solution of potassium

dichromate.

(ii) How many electrons are transferred in this reaction?

__________________________________________________________________ (c) (i) Solid magnesium carbonate is heated strongly.

(ii) How can this reaction be classified?

__________________________________________________________________

4. Silver oxide will undergo a decomposition reaction when heated strongly. (a) Write a balanced equation to show the decomposition of silver oxide. (b) List the steps in an experimental procedure that will allow the determination of the

empirical formula of silver oxide. You may assume that the normal laboratory equipment is at your disposal, and that you have all of the following available: several silver oxide samples, an electronic balance, a crucible with lid, a clay pipe triangle, tongs, wire gauze, Bunsen burner, and tripod. Some sample data from such an experiment is given below.

(i) Suggest what substance is present in the crucible after the strong heating. (ii) Use the data to determine the empirical formula of the silver oxide. (iii) If the silver oxide is not heated sufficiently, explain the effect on the calculated

empirical formula; be specific in your answer.

5. An experiment is carried out in order to determine the water of crystallization within a sample of the hydrated salt, copper(II) sulfate. The following questions relate to that experiment.

(a) As part of the experiment a student performs some of the following actions (this is NOT

a complete list of the whole procedure). For the actions listed, suggest a reason why each is performed.

(b) If the action in (a) (iii) above is not taken, and the experiment is concluded before a

constant mass is recorded, what is the effect on the calculated hydrate formula? (c) As well as the error described in (b) above, certain other errors can occur in this

experiment. Suggest how each of the following may affect the calculated hydrate formula.

(i) When weighing the crucible between heating, the student handles the cool crucible with his fingers rather than using the crucible tongs.

(ii) A tightly fitting crucible lid is used rather than one which is fitted with the lid slightly askew.

(d) Write an equation to summarize the process occurring in this type of experiment using

the hydrated salt, KAl(SO4)2.12H2O.

6. The percentage of sulfate ions in a sample of an unknown salt is to be determined. The following passage details the steps taken during the gravimetric analysis.

The mass of a small sample of the unknown salt is determined with the use of a weighing

boat and an electronic balance. This mass of the solid sample of the salt is completely dissolved in water. A solution of barium chloride is carefully added to the dissolved sulfate solution until no further precipitation is observed. The solution is carefully filtered (using a pre-weighed filter paper) in order to collect the precipitate. The precipitate is then dried in an oven and weighed.

(a) Write a net ionic equation to summarize the reaction being utilized in this experiment

(include phase labels). (b) The data below is collected. Answer the questions that follow;

(i) Calculate the mass of the sample of the unknown salt. (ii) Calculate the mass of the dry precipitate product. (iii) Calculate the mass percent of sulfate ions in the unknown sample. (c) What is the purpose of adding barium chloride solution to the point where no more

precipitate is observed to be formed? (d) What is the effect of failing to dry the precipitate fully on the calculated mass percent of

sulfate ions in the unknown sample?

Answer Key and Explanations

for


Multiple-Choice

Answer Key

Spiral-bound teacher pages may not be shared or reproduced.

Do n

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opyMULTIPLE-CHOICE ANSWER KEY

1. A

2. D

3. E

4. E

5. A

6. B

7. E

8. D

9. B

10. C

11. E

12. C

13. C

14. B

15. D

16. E

17. D

18. A

19. D

20. E

21. C

22. C

23. D

24. D

25. E

26. E

27. E

28. E

29. B

30. A

31. B

32. C

33. D

34. A

35. D

36. D

37. A

38. D

39. B

40. C

41. A

42. B

43. E

44. D

45. D

46. A

47. D

48. B

49. E

50. C

51. D

52. C

53. A

54. C

55. C

56. B

57. C

58. A

59. E

60. B

61. E

62. A

63. D

64. D

65. C

66. C

67. C

68. C

69. A

70. A

71. B

72. D

73. B

74. A

75. C


Multiple-Choice

Answer Explanations


Do n

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opyANSWER EXPLANATIONS

MASSES AND THE MOLE

1. (A) 1 mole. 3 3

3

17.0 g of NH 1 mole of NH molecules17.0 g of NH

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 1 mole of NH3 molecules.

2. (D) 5 moles. 24

23

3.011 x 10 C atoms 1 mole of C atoms6.022 x 10 C atoms

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠= 5 moles of C atoms.

3. (E) 8 moles.

238 8

238 8

6.022 x 10 S molecules 1 mole of S molecules 8 moles of S atoms6.022 x 10 S molecules 1 mole of S molecules

⎛ ⎞⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠⎝ ⎠ = 8

moles of S atoms. 4. (E) I, II and III. All statements are true since CaCO3 has a molar mass of 100.091 g.

5. (A) 2311.0g of Cu atoms 1 mole of Cu atoms 6.022 x 10 Cu atoms

63.55g 1 mole of Cu atoms⎛ ⎞⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠⎝ ⎠. This is

the only choice which cancels units correctly to yield Cu atoms as the final unit. 6. (B) 16.0 g of oxygen molecules. This choice is the only one that represents 0.5 moles. All of the others are equal to 1.0 mole of particles (atoms, molecules or ions). 7. (E) 1.204 x 1024 atoms of carbon is two moles of atoms, as is 48.6 g of Mg. *86.028 g of nitrogen molecules has two moles of molecules, but 4 moles of atoms 8. (D) I and II only. The key word here is ATOMS. I and II represent 1 mole of atoms (as does 12.011 g of C). Statement III represents one mole of Cl2 molecules, which is twice as many atoms. 9. (B) II only. Statement II is incorrect since 120.11 g of C would be 10 moles, i.e. 6.022 x 1024 atoms. 10. (C) 19.0 g of F2 molecules. Choice C is 0.5 moles. All of the other choices contain 2 moles of atoms.

11. (E) 0.5 moles of bromine atoms. 0.5 moles of Br atoms 79.9 g1 mole Br atoms

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

=

39.95 g

12. (C) 26.498 g. 2 3

2 3

0.25 moles of Na CO 105.991 g1 mole of Na CO⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 26.498 g.


Do n

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opy13. (C) III only. Moles are not conserved in a chemical reaction (so statements I and II

are incorrect), but mass IS always conserved. 14. (B) II only. 34.0 g of NH3 is 2 moles of molecules (i.e. 6.022 x 1023 x 2), and 1 mole of NH3 contains one mole of N atoms which have a mass of 14.007 g.

15. (D) 48 g. 23

23

1.5505 x 10 atoms 1 mole of atoms6.022 x 10 atoms

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠= 0.25 moles of atoms. This

has a mass of 12 g, so 1 mole must have a mass of 48 g.


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CHEMICAL EQUATIONS AND STOICHIOMETRY 16. (E) 2Na+ + O2 Na2O2. Charge must be balanced as well as mass. This is not the case in choice E. 17. (D) 4Na + O2 2Na2O. The answer is determined simply from the stoichiometric coefficients of the balanced equation. All others have 1:1 ratios. 18. (A) 2Na+ + O2- Na2O. 1 mole of Na2O has a total of three atoms. All other choices have more than 3. 19. (D) 10. Coefficients are 1, 4, 1, and 4, so the total is 10. 20. (D) 12. Coefficients are 2, 3, 1, and 6, so the total is 12. 21. (C) 6. Balanced equation is C5H12 + 8O2 5CO2 + 6H2O. 22. (C) 1.5. Coefficients are 2, 3, 6, 1, and 1. Since the relevant ration is 2:1 half as much NO is formed from the initial amount of NCl3. 23. (D) II and III only. Coefficients are 2, 3, 2, and 2. 24. (D) 5. Coefficients are 1, 8, 5, 5, and 4. 25. (E) 9. Coefficients are 1, 3, 14, 3, 2, 7, so three times as much Sn2+

(aq) is need as Cr2O7

2-(aq).

26. (E) I, II and III. Statement I is true since it yields the correct units and has the correct ratios taken from the balanced equation. Statement II is true since the coefficients for both are the same. Statement III is true and can be seen by inspection. 27. (E) I, II and III. Coefficients are 2, 1, 1, and 2. All statements are true and can be seen by inspection. 28. (E) I and III only. The correct whole number coefficients are 2, 3, 2, 2 (statement I), and statement III’s coefficients are in the same ratio. 29. (B) CH4 + 2O2 CO2 + 2H2O. By inspection these are the correct formulae and the equation is balanced. 30. (A) 9, 2, 6, 1, 3. Can be seen by inspection.


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STOICHIOMETRY APPLICATIONS 31. (B) CH2. Empirical formula is the simplest whole number ratio and 4 and 8 and both divisible by 4. 32. (C) CH3. In this formula 12.0 g of C are present in a total of 15.0 g.

12 x 100% = 80%15⎛ ⎞⎜ ⎟⎝ ⎠

. This is also true of choice E, but that is NOT an empirical

(simplest whole number) formula. 33. (D) CH4. In this formula 4.0 g of H are present in a total of 16.0 g

4 x 100% = 25%16⎛ ⎞⎜ ⎟⎝ ⎠

.

34. (A) ( )4.345 g - 2.456 g

x 1002.976 g

⎛ ⎞⎜ ⎟⎝ ⎠

.

Percentage yield is calculated thus: Actual Yield x 100%Theoretical Yield

⎛ ⎞⎜ ⎟⎝ ⎠

and the actual yield can

be calculated by subtracting the mass of the filter paper from the mass of the product + filter paper. 35. (D) 36% Ca and 64% Cl. The formula of calcium chloride is CaCl2 which has 40 g of Ca for every 71 g of Cl in every one mole.

% of Ca = 40 x 100% = 36%111⎛ ⎞⎜ ⎟⎝ ⎠

and so the % of Cl must be 64%.

36. (D) C2H2, C4H4, C6H6. All of these formulas have an empirical formula CH (i.e. a 1:1 ratio of C’s to H’s). 37. (A) III only. Statements I, II and IV all have coefficients that can be simplified further so are not empirical formula. 38. (D) II and III only. Oxygen is the limiting reagent so is used up completely. Hydrogen is in excess so will be left in the reaction vessel at the end of reaction along with the product. 39. (B) 20.0%. 246 g of nitrobenzene is 2 moles. From the equation we can see that there is a 1:1 of nitrobenzene to the product so we should get 2 moles of product. The product has a molar mass of 137 g mol-1, so we should get 274 g of product. 54.8 g is 20% of 274 g.


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opy40. (C)

40 12 48 There are 40 g of Ca, 12 g of C and 48 g of O in every 100g of CaCO3. 41. (A) 11.0 g. 50 g of CaCO3 is 0.5 moles. From the equation we can see that there is a 1:1 of CaCO3 to CO2 so we should get 0.5 moles of CO2. CO2 has a molar mass of 44 g mol-1, so we should get 22 g of CO2. 50% of 22 g is 11 g. 42. (B) NaOH. From the balanced equation we see that there should be four times as much NaOH as CH3COCH3, and three times as much I2. 20 moles of NaOH is less than the required 24, so it is the limiting reactant. 43. (E) None of the statements are true. Statement I is untrue since the mass of N in NO is 14 g and the mass of O is 16 g – i.e., NOT 50:50. Statement II is untrue since the empirical formula (simplest whole number ratio) is 1:2, and N2O5 and NO2 ARE in their simplest whole number forms so they have different empirical formulas. 44. (D) SCl2 and S3Cl6. In both of these formulae there are twice as many chlorine atoms as sulfur atoms meaning their mass ratios will be the same. 45. (D) II and III only. No process can yield more than 100% so statement I is false. Statement II is a fact and ALL products depend upon limiting reactants by definition.


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TYPES OF REACTIONS 46. (A) Precipitation. Silver ions will precipitate the bromide ions in the net ionic equation, Ag+

(aq) + Br-(aq) AgBr(s).

47. (D) Decomposition. The reaction is summarized thus: MgCO3 MgO + CO2 48. (B) Single Replacement. The reaction is a hydrogen displacement from water by a reactive metal. The equation is: 2K + 2H2O 2KOH + H2. 49. (E) I, II, III and IV. The single metal displacement reaction can be summarized thus: Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq). Iron(II) sulfate is produced along with copper metal. The Fe is oxidized and the Cu is reduced. 50. (C) I and II only. Reaction in I would precipitate insoluble AgBr; reaction in II would precipitate insoluble PbI2; reaction in III is a hydrogen displacement from water by a reactive metal and yields no precipitates. 51. (D) I and III only. This reaction is a displacement of hydrogen by a metal from an acid, and can be summarized by this net ionic equation: Zn(s) + 2H+

(aq) Zn2+(aq) + H2(g).

52. (C) I, III and IV only. Reducing agents cause reduction by giving electrons away. In the process they are oxidized. A redox process must have an electron “source” and a “sink” – the electrons have to come FROM somewhere and have to go TO somewhere. All equations must balance in terms of charge AND mass. 53. (A) It gains electrons and Mn2+

(aq) is a product. Manganate(VII) (or permanganate) ions are oxidizing agents and as such gain electrons and are reduced in redox reactions. In acid solution the Mn species will tend to gain 5 electrons. 54. (C) The corresponding oxide and CO2. A typical reaction is CaCO3 CaO + CO2 where Ca can be replaced by any group 2 metal. 55. (C) Synthesis. The reaction can be summarized thus: S(s) + O2(g) SO2(g). The combination of elements to form (make) a compound can be classified as a synthesis (combination). 56. (B) Solid copper(II) hydroxide and a solution that contains sodium and sulfate ions. The net ionic reaction is Cu2+

(aq) + 2OH-(aq) Cu(OH)2(s). Sodium and sulfate ions

remain in solution and therefore are spectators. 57. (C) Mg3(PO4)2(s). The net ionic equation is: 3Mg2+

(aq) + 2PO43-

(aq) Mg3(PO4)2(s). The sodium and chloride ions remain in solution, are spectators and as such do not appear in the net ionic equation.


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opy58. (A) The strong heating of solid calcium carbonate in the absence of oxygen. The

reaction is CaCO3(s) CaO(s) + CO2(g). One reactant yields two or more products meaning a decomposition reaction has taken place. 59. (E) Li3N(s). The reaction is a combination or synthesis where two elements combine to form a new compound. There is no water present so the product does not dissociate into its ions, despite the fact it is soluble: 6Li(s) + N2(g) 2Li3N(s). 60. (B) No reaction would occur. All group 1 salts (including potassium salts) are soluble, so no net ionic reaction takes place – all ions are spectators.


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LABORATORY APPLICATIONS 61. (E) Hygroscopic. This is a definition. 62. (A) Anhydrous salt. When water of crystallization is driven off by heating, the residue left behind is said to be anhydrous, or “without water”. 63. (D) Constant mass. When determining their formula, hydrated salts must be heated to remove ALL of the water of crystallization. To ensure this, they must be heated to a “constant mass”, i.e. heated until no more water can be removed. 64. (D) 7. The mass of water released = 24.6 – 12.0 = 12.6 g.

Moles of water = 12.6g of water 1 mole of water18.0 g of water

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.7 moles.

Mass of residue = 12.0 g.

Moles of residue = 4 4

4

12.0g of MgSO 1 mole of MgSO120 g of MgSO

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.1 moles.

Molar ratio of MgSO4 to H2O = 1:7. 65. (C) Reheat the sample to a constant mass and then carry out the procedure in (A) with the new mass rather than 4.18 g. When determining their formula, hydrated salts must be heated to remove ALL of the water of crystallization. To ensure this, they must be heated to a “constant mass,” i.e., heated until no more water can be removed. Essentially (A) is the correct procedure, but one must ensure that constant mass has been achieved. 66. (C) I and II only. Bromide ions will also precipitate out with silver ions, meaning that some precipitate is NOT AgCl as one may assume. Failure to dry the precipitate will give a mass of AgCl that is artificially high. Potassium ions will not precipitate chloride ions. 67. (C) 31.0. In one molecule or formula unit of a compound, there must be a whole number of T atoms. 31 is a common factor to the masses of T present in the four different compounds meaning there are different (whole) numbers of T atoms in each of the compounds. 68. (C) 48 g mol-1. Two moles of oxygen atoms have a mass of 32 g. If 32 g is equivalent to 40%, then 100% has a mass of 80 g. This makes the mass of J = 80 -32 g = 48 g.


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opy69. (A) XO. The difference in mass between the crucible and its contents before and after

heating must be the mass of oxygen that has reacted. Converting this to moles via this calculation:

( )Mass of O atoms = 30.282 - 30.123g 1 mole of O atoms16.0 g of O atoms

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.01 moles of O.

This is in a 1:1 ratio with X, so the formula is XO. 70. (A) I only. Barium ions will precipitate the sulfate ions. Sodium and magnesium ions will not, since their sulfates are soluble. 71. (B) The residue after heating will be recorded as being smaller than the correct value and as a result the percentage of water calculated will be too large. Loss of salt will result in smaller mass being recorded for the residue, which in turn will report a larger value for the mass of water lost. BOTH of these factors contribute to the percentage of water being reported too high. 72. (D) I and II only. Bromide and chloride ions will precipitate with the silver ions but nitrate ions will not.

73. (B) FeCl2. 2 2

2

71.0 g of Cl 1.00 mole of Cl71.0 g of Cl

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 1.00 mole of Cl2 = 2 moles of Cl.

Since this reacts with 1 mole of Fe the formula is FeCl2. 74. (A) MgSO4.7H2O. Heating one mole of this salt to a constant mass will cause 126 g of water to be lost, leaving 120 g of anhydrous salt, i.e. over half of the mass will be lost. All of the other salts will lose less that 50% of their total mass since the anhydrous parts have a greater mass than the water of crystallization present. 75. (C) III only. The mass ratios are as follows: 24:16, 46:16, 46:32 46:16, 46:32, 56:16 46:16, 24:16, 54:48


Free-Response Answers and

Scoring Guides


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opyFREE-RESPONSE ANSWERS

REACTIONS AND STOICHIOMETRY 1. (a) Two “best fit,” straight lines. One through the ascending points on the left,

and one through the descending points on the right. (2 points: 1 point for each line)

(b) 30 mL of silver nitrate, 10 mL of unknown phosphate solution. Some slight

variation allowed here. The volume of the silver nitrate should be where the two straight, lines cross and form an apex. (2 points: 1 point for each volume)

(c) 3AgNO3 + X3PO4 Ag3PO4 + 3XNO3. (2 points: 1 point for correct formulas,

1 point for balancing) (d) 3Ag+

(aq) + PO43-

(aq) Ag3PO4(s). (2 points: 1 for correct phases and 1 for eliminating spectator ions)

(e) Unknown phosphate is limiting. Up to the apex the silver nitrate was

limiting (adding more of it continued to produce greater mass of precipitate). After the apex the silver nitrate is in excess (i.e., the phosphate is limiting) since adding greater amounts of it does not produce greater masses of precipitates. (1 point for identifying the unknown phosphate as the salt; 1 point for the explanation)

(f) Temperature change, conductivity change, absorption change etc. (1 point)


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REACTIONS AND STOICHIOMETRY

2. (a) (i) C4H9OH + 6O2 4CO2 + 5H2O. (4 points: 1 point for correct reactants, 2 points for correct products, 1 point for balancing)

(ii) 2-butanol (any isomer of 1-butanol accepted ). Please note that organic

nomenclature is required for this part of the question. You may want to either leave this for the review time at the end of the course, or give a brief introduction at this early juncture in the curriculum. (1 point)

(b) (i) Pb2+ + CrO4

2- PbCrO4. (4 points: 1 point for correct reactants, 2 points for correct products, 1 point for balancing)

(ii) Yellow/Orange. (1 point) (c) (i) Br2 + 2I- I2 + 2Br-. (4 points: 1 point for correct reactants, 2 points for

correct products, 1 point for balancing) (ii) Halogen displacement or displacement or redox or single displacement,

or reduction-oxidation. (1 point)


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3. (a) (i) Cs2O + H2O 2Cs+ + 2OH-. (4 points: 1 point for correct reactants, 2 points

for correct products, 1 point for balancing) (ii) Red litmus turns blue. Hydroxides are basic. (1 point) (b) (i) Cr2O7

2- + 6I- + 14H+ 3I2 + 2Cr3+ + 7H2O. (4 points: 1 point for correct reactants, 2 points for correct products, 1 point for balancing)

(ii) 6 electrons are exchanged. Half reactions are: (work need not be shown) Cr2O7

2- + 14H+ 2Cr3+ + 7H2O + 6e-

2I- I2 + 2e-

(1 point) (c) (i) MgCO3 MgO + CO2. (4 points: 1 point for correct reactants, 2 points for

correct products, 1 point for balancing) (ii) Decomposition reaction. (1 point)


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4. (a) 2Ag2O 4Ag + O2. Decomposition splits the compound into its elements. (1 point)

(b) Many possible answers and styles here, but the essentially points are scored for

the following: • Clean and dry the crucible and weigh it. (1 point for the mass of the clean,

empty crucible) • Add a known mass of silver oxide. (1 point for weighing the crucible with

the compound) • Heat carefully at first followed by more vigorous heating until no further

reaction occurs. (1 point for heating) • Allow crucible to cool and reweigh. (1 point for weighing the crucible with

the product) (c) (i) Silver metal. (1 point for the correct product) (ii) Mass of silver oxide = 24.821 – 21.986 g = 2.835 g Mass of silver at end of experiment = 24.627 – 21.986 g = 2.641 g Mass of oxygen released = 2.835 – 2.641 g = 0.194 g

Moles of Ag atoms = 2.641 g of Ag 1 mole of Ag108g of Ag

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.0245 moles

Moles of O atoms = 0.194 g of O) 1 mole of O16 g of O

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.0121 moles

Mole ratio Ag:O = 0.0245:0.0121 = 2:1 or Ag2O. (4 points: 1 point for the correct mass of silver, 1 point for the correct mass of oxygen, 1 point for converting these masses to moles [point awarded even if incorrect masses were used], 1 point for using moles as a ratio to find empirical formula)

(iii) It will appear that less oxygen is released, and the moles of O will be too small. (1 point for explanation and effect on the formula)


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5. (a) (i) This is to ensure that all potential impurities have been removed, and that the masses recorded are strictly ONLY those of the hydrated salt and crucible. (1 point for explanation)

(ii) To avoid splattering and the loss of salt. (1 point for explanation) (iii) To ensure that all of the water of crystallization has been driven off. At

this point the mass will not undergo any further change, and is said to be a “constant mass.” (1 point for indicating that this is the signal that the reaction is complete)

(b) The mass of water will be recorded as being too small, meaning the moles of

water will be calculated as being too small in the final formula. (2 points: 1 point for the effect on the formula and one point for the explanation why.)

(c) (i) Grease from fingers may add to the total mass of the crucible and its

contents, which would suggest that less water has been removed than actually has been. This would lead to a calculation of too few moles of H2O in the final analysis. (2 points: 1 point for effect on the formula and 1 point for explanation; students may consider the issue of safety, handling a hot crucible, and possibly spilling the salt)

(ii) All of the water may not escape, meaning the mass of water lost

calculated would be too small. This would lead to a calculation of too few moles of H2O in the final analysis. (2 points: 1 point for effect on formula and 1 point for the explanation)

(d) KAl(SO4)2.12H2O KAl(SO4)2 + 12H2O. (1 point for the equation)


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6. (a) Ba2+(aq) + SO4

2-(aq) BaSO4(s). (2 points: 1 point for the product and 1 point for

reactants in correct form) (b) (i) 4.235 – 3.167 g =1.068 g. (1 point for work and answer [labels of what is

subtracted is acceptable]) (ii) 3.257 – 2.208 g = 1.049 g. (1 point for work and answer [labels of what is

subtracted is acceptable])

(iii) Moles of BaSO4 ppt. = 41 mole of BaSO1.049 g233 g

⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.00450 moles

Moles of SO42- is the same (since 1:1 ratio) = 0.00450.

Mass of SO42- =

2-4

2-4

0.00450 moles of SO 96 g1 mole of SO⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠ = 0.432 g

% of sulfate ions = 0.432 g x 100%1.068 g⎛ ⎞⎜ ⎟⎝ ⎠

= 40.5%. (4 points: 1 point for moles of

barium sulfate, one point for moles of sulfate, 1 point for mass of sulfate ions, 1 point for mass percent)

(c) To ensure that ALL of the sulfate ions have been precipitated out of

solution. (1 point) (d) The mass of BaSO4 would be recorded too high, and as a result so would its

moles. This would make the moles of sulfate ions too high as well, and their mass would also be too high. In turn, this would produce a mass percent of sulfate ions that was too high. (2 points: 1 point for answer, 1 point for explanation)


Contributors Series Editor David Emmerson is a veteran teacher with more than twenty years’ experience teaching AP Chemistry. He has previously worked on developing an online AP Chemistry course. He holds a B.S. in Biology from Cornell University and an M.A. in Science Education from the State University of New York Writer Adrian Dingle is a chemistry educator with 18 years’ experience and is creator of the award-winning chemistry Web site www.adriandingleschemistrypages.com. He holds a B.Sc. (Hons.) Chemistry and a Postgraduate Certificate in Education from the University of Exeter, England.

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