chemical formulas and composition stoichiometry chapter 2
TRANSCRIPT
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Chemical Formulas and Composition Stoichiometry
Chapter 2
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Stoichiometry
• Describes the ___________ relationships among elements in compounds (composition stoichiometry) and among substances as they undergo chemical changes (reaction stoichiometry).
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Dalton’s Atomic Theory• An element is composed of extremely small, indivisible
particles called _____.• All atoms of a given element have ______ properties
that are ______.• Atoms cannot be _____, ______, or ______ into atoms
of another element. • ______ are formed when atoms of different elements
combine with one another in small whole-number ratios.
• The relative _____ and ____ of atoms are constant in a given compound.
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Atoms and Molecules
• We know what atoms are and the fundamental particles that compose them.
• _______ is the smallest particle of an element or compound that can have a stable, ______ existence.– This means that if a molecule contains only one
type of atom it is also an element.
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Atoms and Molecules
• Monatomic – contains only ___ atom– Na and He For these elements, an atom and a molecule are
the same.
• Diatomic – molecules that contain only __ atoms– H2, O2, and all halogens
• Polyatomic – molecules than contain ______ atoms– P4, S8, and C60
Allotropes – different _____ (geometry or number of atoms) of the same element
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Compounds• Compounds are substances that contain two
or more elements in fixed proportions– Compounds containing only covalent bonds
• The atoms in a compound _________ electrons.• These compounds are also called molecules since
they exist as individual particles.
– Compounds containing ionic bonds• These compounds are held together by electrostatic
interactions.• These are not molecules since they do not exist as
individual particles.
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Covalent Compounds
• The chemical formula represents the ____ present and the ______ in which the atoms of the elements occur. – Table 2.2 Show a few models to illustrate that all
atoms are held together by covalent bonds.• H2O and C4H10 (models)• Notice that the formula is not necessarily the simplest ratio
• _________ compounds are largely composed of C-C,C-H, and C-O bonds.– Ethanol and benzene
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Covalent Compounds
• Inorganic compounds largely do not contain _____ and/or ____ bonds.– H2O and NH3
• Law of Definite Proportions – different pure samples of a compound always contain the same elements in the same proportion by mass– _______ of atoms can also affect the properties of
compounds even if they have the same chemical formula.
Demo: John Cullen’s demonstration
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Compounds Containing Ionic Bonds
• Held together by __________ between ions• They do not exist as individual units
– Collection of a large number of ions
– The formula represents the ______ of ions in the compound
• Each positively-charged species is surrounded by negatively-charged species (and vice-versa)– DEMO: NaCl structure
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Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Positive and negative ions will combine in such a way to make the ionic compound _____.
• Binary Ionic Compounds– You already know how to write the formulas.
• Write the formula for calcium bromide and lithium sulfide
– Naming a binary compound from the formula• Cation – name of the metal• Anion – drop the last portion of the name and add ‘ide’• Name AlBr3 and BeI2
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Common Ions
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More Polyatomic Ions
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Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Ionic compounds containing a cation that can have multiple charges– The charge on the cation has to be specified– What is the charge on FeCl3 and FeCl2?
• In most cases, the charge can be determined by the negatively-charged ion
• Write the names for these compounds?
– Write the formulas for copper(II) sulfide and iron(III) oxide.
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Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Ionic compounds containing polyatomic ions
• The atoms in polyatomic ions are held together by _________.– Write the Lewis structure for SO3
2-
• The polyatomic ions and the oppositely-charged ions are held together in the solid by _________
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Writing Formulas and Names for Ionic Compounds (Table 2-3)
• Write the names for (NH4)2S and K2(SO4).
– Notice that the charges are fixed and given on Table 2-3 for the polyatomic ions.
• Write the formulas for magnesium nitrate and lithium carbonate.– If more than one polyatomic ion is needed
parentheses are placed around it
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Atomic Weight
• The atomic weight is based on the atomic mass unit.– One amu is _____ the mass of a carbon-12
atom
• The atomic weight, which is given on the periodic table, is a _______ of all the naturally occurring isotopes of that element
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The Mole
• A mole describes a ________ of objects, particles, or atoms. This is similar to other measurements that describe a particular quantity.– 1 dozen = 12 items– Gross = 144 items– 1 mole = _____________ (much larger)
• Avogardo’s number
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The Mole
• A mole is the amount of substance that contains as many entities as there are atoms in 12.0 grams of carbon-12.
• One mole of H2O would contain 6.02 1023 molecules of H2O.
• The mass of one mole of atoms of an element is equal to its atomic weight (periodic table) in grams.– This is called the molar mass (units ________).
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The Mole
• Iron (Fe)
• These conversion factors will be used extensively!!• Calculate the mass of a single Mg atom.• Calculate the number of atoms in 1.40 10-15 moles
of Cu.• Calculate the number of moles in 85.3 grams of Al.
atomsFemole1atoms1002.6
andFeg847.55atomsFe1002.6
andFemole1Feg847.55 2323
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Molar Mass
• The ________ of a substance is the sum of the atomic weights of all the atoms in the formula. Expressed in amu.
• _________ can be used if the compound is composed of molecules.
• Molar mass is _________ to the formula weight of a compound. Expressed in grams– Indicates how many grams in ______ of the compound
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Molar Mass
• This molar mass of a substance will contain 6.02 1023 formula units
• What is the molar mass of Ca(NO3)2?• Calculate the number of C3H8 molecules in
75.4 grams of C3H8
• Calculate the number of millimoles in 0.234 g of oxalic acid (CHOO)2
OHmassmolarOHunitsformula1002.6
andOHunitsformula1002.6
OHmole1and
OHmole1OHmassmolar
OHFW2
223
223
2
2
22
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More Problems
• The chemical formula indicates the relative number of ______ and _____ in a compound.– Shown by the ________ C3H8
• How many formula units in 2.5 moles of CaCl2? How many Ca2+ and Cl- ions in 2.5 moles of CaCl2?
• Calculate the number of oxygen atoms in 54.2 grams of sucrose, C6H12O6.
• Calculate the number of moles of hydrogen in 54.2 grams of sucrose.
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Percent Composition
• The mass percent of each element present in a compound is simply determined by dividing its mass by the molar mass of the compound multiplied by 100%.
• Let’s do H2O. What is the percent composition of oxygen and hydrogen?
• What is the percent composition of carbon and hydrogen in C3H8?
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Percent Composition
• The percentages must add up to _____. Any pure compound will contain the same percentages of each element (____ __ _____ ________).
• One more – What is the percent composition of Fe in Fe2(SO4)3?
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Determining Empirical Formulas from Percent Composition
• Empirical formula (or simplest formula) is the __________ ratio of elements in a compound.
• Molecular formula is the ________ of atoms present in a compound containing covalent bonds.
The empirical formula and molecular formula are not necessarily the same.– Benzene (show structure) Molecular vs. empirical– Hydrogen peroxide (show structure) Molecular vs.
empirical– Sucrose
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Determining Empirical Formulas from Percent Composition
• A compound contains 24.74% K, 34.76% Mn, and 40.50% O. What is the empirical formula of this compound?– Step 1: Consider how many grams of each element
would be present in 100.0 grams.– Step 2: Convert to moles for each element.– Step 3: Obtain smallest whole-number ratio by
dividing by the smallest mole number.– Step 4: If necessary, convert fraction to whole numbers
by multiplying an integer.
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Determining Empirical Formulas from Percent Composition
• A compound contains 7.537% H, 26.185% N, and 66.278% Cl. What is the empirical formula?
• A sample of a compound contains 6.541 grams of Co and 2.369 grams of O. Find the simplest formula.
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Combustions Train – Analysis of Hydrocarbons
• Combustion trains are used to perform quantitative carbon-hydrogen analysis.
• A known mass is burned in a stream of oxygen.– Carbon is converted to CO2
– Hydrogen is converted to H2
Weight increase in the absorber region is measured.
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Combustions Train – Analysis of Hydrocarbons
• Determine the mass of carbon in the compound– Use the conversion factor
• Determine the mass of oxygen in the compound– Use the conversion factor
Why do we use these conversion factors?
Which units will cancel?
2COg01.44Cg01.12
OHg02.18Hg016.2
2
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Combustions Train – Analysis of Hydrocarbons
• A 0.2815-gram sample of pure hydrocarbon was burned in a combustion train to produce 0.4362 grams of H2O and 0.8527 grams of CO2. Determine the masses of C and H in the sample and the percentages of these elements in this hydrocarbon.
I’ll give you one on your chapter problems.
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Combustions Train – Analysis of Hydrocarbons
• Compounds that contain C, H, and O.
• A 0.3615-gram compound containing on C, H, and O was burned in a combustion train to produce 0.8219 grams of CO2 and 0.3364 grams of H2O. Determine the masses of C, H, and O in the sample and the percentages of these elements in this hydrocarbon.
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Determining Molecular Formulas
• Percent composition only yields empirical formulas.– Determine the empirical formulas for the last two
problems.
• In order to determine the molecular formula, both the _______ and the _________ must be known.
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Determining Molecular Formulas
• The molecular formula is an integer multiple of the simplest formula.– C6H6 and C6H12O6
– Molecular formula = n simplest formula– It follows that;
• Molecular weight = n simplest formula weight (rearrange to solve for n)
• The molecular weight of the compound in the first problem was determined to be 58.123 amu. What is the molecular formula?
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Determining Molecular Formulas
• The molecular weight of the compound in the second problem was determined to be 174.240 amu. What is the molecular formula?
Have fun!!!
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More Fun with Conversions
• What mass of ammonium phosphate, (NH4)3PO4 would contain exactly 15.00 grams of N?
• What mass of sucrose, C6H12O6, would contain the same mass of oxygen as is contained in 25.1 grams of ammonium phosphate?
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Solid Hydrates
• Solid hydrates are crystalline solids that contain _______ which can be generally removed by heating.– DEMO: CuSO45H2O The water can be removed by
heating. – Every mole of CuSO45H2O contains _____ of water.
• An unidentified hydrate of magnesium sulfate is heated, MgSO4xH2O. The mass before heating was 78.30 grams. The new mass after heating is 38.24 grams. What is the formula of the hydrate?
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Purity of Samples
• Samples are never 100% pure. Percent purity is used to specify the purity of a particular sample.
• In many cases, 100 grams of sample can be used for convenience.
• A bottle of Na3PO4 is 98.3% pure. What are the masses of Na3PO4 and impurity in 251 grams of sample?
• Calculate the number of moles of C2H4Cl2 in 58.2 g of 92.1% pure C2H4Cl2.
%100sampleofmass
cetansubspureofmasspurity%
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Solving Problems
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Let’s Do Some More Problems