AP Unit 1 PPTSTOICHIOMETRY

Video AP1.1Moles and Molar Mass

Objectives

You should already be able to…• Identify elements by name and symbol• Calculate molar masses of elements and

compoundsBy the end of this video you should be able to….• Use dimensional analysis to calculate the

number of a moles of particles in a sample.• Use dimensional analysis to calculate the

mass of a given sample.

Stoichiometry

The Mole represents 6.02x1023 particles such as atoms and molecules.

The Mole = 6.02x1023

The mole is based on the fact that 12 grams of Carbon-12 has a mole of atoms.

Molar Mass

Molar Mass (aka gram formula mass or molecular mass) is the mass of one mole of a substance.

Element’s molar masses are reported on the periodic table.

Molar Mass Examples: Elements

1. What is the molar mass of iron?

2. What is the molar mass of copper?

55.85 g/mol

63.55 g/mol

Molar Mass Examples: Compounds

1. What is the molar mass of water?

H2O = 2(1.008) + 16.00 =

18.016g/mol

2. What is the gram-formula-mass of calcium chloride?

CaCl2 = 40.1 + 2(35.5) =

111.1 g/mol

Dimensional Analysis

1. Calculate the mass of 6.70 moles of Neon.

Dimensional Analysis

2. Calculate the mass of 0.023moles of NH3.

Dimensional Analysis

3. Determine the number of moles in 0.567grams of Helium.

Dimensional Analysis

4. Determine the number of moles in 8.0 grams of CCl4.

Check Your Understanding

Can you calculate the moles of an element or compound using dimensional analysis?

Don’t cheat yourself of understanding… take out a pen and paper and calculator and really try to set this up!

DA pointers summarized

Start with the number given as a numerator over 1.

MULTIPLY by a new fraction Place a conversion with the same

unit on the denominator of the new fraction.

Multiply numerators, divide denominators. (maybe pause

and copy all this?)

You must be able to…

• Use dimensional analysis to calculate the number of a moles of particles in a sample.

• Use dimensional analysis to calculate the mass of a given sample.

Video AP 1.2Molar Volume and Avogadro

Objectives

You should already be able to…• Identify elements by name and symbol• Use dimensional analysis to calculate moles and

masses of elements and compoundsBy the end of this video you should be able to….• Use dimensional analysis to calculate the number

of a moles of particles in a sample given the number of atoms or molar volume.

• Use dimensional analysis to calculate the molar volume or number of particles of a given sample.

Mole Road Map

mass• Gram

formula mass

volume

• Gases at STP: use 22.4L

particles

• Atoms or molecules: use 6.02x1023

Examples

1. Calculate the number of molecules in 4.37 moles of Lithium sulfide.

2. Calculate the number of atoms in question 1.

Examples

3. What is the volume of 7.89 moles of He gas at STP?

Examples

4. How many moles of atoms are present in 79.80 Liters of diphosphorous pentoxide gas at STP?

Examples

5. How many moles are present in 5.67x1024 molecules of H2O(l)?

Check Your Understanding

Can you calculate the moles, liters, or number of particles of an element or compound using dimensional analysis?

Don’t cheat yourself of understanding… take out a pen and paper and calculator and really try to set this up!

THINK

Why do chemists use moles to measure substances? Why aren’t

grams, liters and molecules enough?

You must be able to…

• Use dimensional analysis to calculate the number of a moles of particles in a sample given the number of atoms or molar volume.

• Use dimensional analysis to calculate the molar volume or number of particles of a given sample.

Video AP 1.3Stoichiometry

Objectives

You should already be able to…• Use dimensional analysis to calculate the

number of a moles of particles in a sample.• Use dimensional analysis to calculate the

mass, volume or number of particles of a given sample.

By the end of this video you should be able to….• Use dimensional analysis to calculate

multistep mole problems.

Examples

1. Calculate the volume of 13.80 grams of Ar gas at STP.

Examples

2. Calculate the mass of 2.5L of oxygen gas at STP.

Examples

3. Calculate the number of atoms of P in 120.0 grams of P2O5.

Check Your Understanding

Can you calculate the moles of an element or compound using dimensional analysis?

Don’t cheat yourself of understanding… take out a pen and paper and calculator and really try to set this up!

Reactions

Many AP questions will give you information about a reactant used in a reaction and ask you about the product formed. Use the balanced equation to create a mole ratio to convert between moles of substances.

Examples

4. Water decomposes into its elements when electrified at high voltages. If 24.0 grams of water is completely decomposed, how many grams of oxygen will be produced?

Examples

5. Lithium and magnesium chloride react according to the equation below. Calculate the number of atoms of magnesium that form when 2.50 moles of lithium are used.

Li + MgCl2 LiCl + Mg

Check Your Understanding

Can you calculate the moles of an element or compound using dimensional analysis?

Don’t cheat yourself of understanding… take out a pen and paper and calculator and really try to set this up!

You must be able to…

• Use dimensional analysis to calculate the hardest AP stoichiometry problems

Video AP 1.4Theoretical and Percent Yield

Objectives

You should already be able to…• Use dimensional analysis to calculate the

number of a moles of particles in a sample.• Use dimensional analysis to calculate the

mass, volume or number of particles of a given sample.

By the end of this video you should be able to….• Use dimensional analysis to calculate the

theoretical and percent yield of a product in a reaction.

Limiting Reagents

Given:

20 11 36

1 sandwich = 2 + 1

+3

1. How many sandwiches can I make if I used all of the bread with unlimited other resources?

2. How many sandwiches can I make if I used all of the cheese with unlimited other resources?

Limiting Reagents

Given:

20 11 36

1 sandwich = 2 + 1

+3

3. How many sandwiches can I make if I used all of the ham with unlimited other resources?

4. How many sandwiches can I make if I used only what is given?

5. What ingredient was the limiter? Why?

Limiting Reagents Examples1. S + 3F2 SF6

Suppose you have 4 moles of sulfur and 2 moles of fluorine, which is the limiter and which is in excess? How many moles of SF6 can be produced?

4S produces 4 SF6

2F2 produces 0.6 moles of SF6 (F2 Limiter!)

Limiting Reagents Examples

2. 2NH3 + CO2 (NH2)2CO + H2OIf 637.2 grams of ammonia react with 1142 grams of carbon dioxide, find the limiter and the amount of urea produced.

Limiting Reagents Examples

3. In question 2, how much of the excess is left over after the reaction goes to completion?

Limiting Reagents Examples

4. 5Ca + V2O5 5CaO + 2V1.96x103 grams of calcium react with 1.54x 103 grams of V2O5. Find the limiter and the amount of V produced.

5. If 803 grams of V are actually made calculate the percent yield.

Check Your Understanding

Can you calculate the theoretical and percent yield of a product given a reactant?

(I hope you have these two words written with definitions on your notes)

You must be able to…

• Use dimensional analysis to calculate the theoretical and percent yield of a product in a reaction.

Video AP 1.5Percent Composition, Empirical and Molecular Formulas

Objectives

You should already be able to…• Calculate the percent composition of an element

in a compound.• Reduce formulas to find the empirical formula.• Calculate the molecular formula of a compound.By the end of this video you should be able to….• Calculate the empirical and molecular formula

of a compound given the percent by mass of its elements.

Percent Composition

% By mass: mass part x 100 total mass

1. Find the % by mass of phosphoric acid.

H3PO4 3(1.0) x 1003(1.o)+32.1+4(16.0)

= 3.02%

Example 2

Find the percent by mass of Ca in Ca(OH)2.

40.0 x 10040.0+2(16.0)+2(1.0)

= 54.1%

Percent Composition Examples

A sample of a substance containing only magnesium and chlorine was tested in the laboratory and found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what is the mass of the magnesium? 0.745 (190.2) =

141.7 grams

Check Your Understanding

Can you calculate the percent composition of a sample?

Empirical Formulas

Empirical formula refers to any molecular formula in it’s reduced form.

Are these empirical? If not, reduce them:

1. C4H8

2. C6H12O6

3. N3O6

4. Na2(OH)2

1.CH2

2.CH2O3.NO2

4.NaOH

Molecular Formulas

Molecular Formulas are some multiple of the empirical formula. If the empirical formula is CH4 a

molecular formula could be CH4, C2H8, C3H12 etc.

A compound whose empirical formula is NH3 has a mass of 34 g/mol. What is the empirical formula?

NH3 = 17.0 g/mol 34/17 = 2

2NH3 = N2H6

Steps:

Find the mass of the empirical formula.

Divide the mass given by the empirical mass.

Distribute your answer through the empirical formula.

If a compound has a mass of 45 g/mol and an empirical formula of CH3, what is the molecular formula?

CH3 = 15 g/mol 45/15 = 33CH3 = C3H9

Copy this?

Calculating Empirical Formulas From Percent Composition

Rules: Find the empirical formula of a compound with 40.92% C, 4.58% H and 54.50% O by mass.

1. Convert the % to grams, assuming the total mass of the sample is 100 grams.

40.92g C, 4.58g H, 54.50g O

2. Convert mass to moles, using atomic masses.

40.92/12 = 3.40 moles

C4.58/ 1 = 4.58 moles H54.50/16 = 3.4o moles O

3. Divide each element’s moles by the smallest number of moles.

4. Find a whole number ratio.

1 mole C, 1.5 mole H, 1 mole O

C2H3O2

Now try these…

1. A sample of nitrogen and oxygen contains 1.52 g N and 3.47g O and a molar mass of about 90 grams. Find the empirical and molecular formulas.

Examples

2. Find the empirical and molecular formula of a 30 gram sample with 6.44 grams of Boron and 1.803 grams of hydrogen.

Multiple Choice

3. A sample of a compound contains 47.0% silicon and the rest is oxygen. If the total mass is 180 grams, what is a possible set of empirical molecular formulas of the compound?

a. SiO2 and SiOb. Si4O4 and SiOc. Si2O4 and SiO2

d. Si3O6 and SiO2

Check Your Understanding

Can you determine the empirical and molecular formulas of a compound?

You must be able to…

• Calculate the empirical and molecular formula of a compound given the percent by mass of its elements.

AP Video 1.6Waters of hydration

Objectives

You should already be able to…• Identify hydrate formulas.• Calculate the molar mass of a hydrate.• Calculate percent by mass of a sample.By the end of this video you should be able to….• Explain the hydrate lab set up in detail.• Calculate the moles of water in a

hydrated sample.

Hydrates

Hydrates contain a specific number of moles of water attached to a molecule: CuSO4

.5H2O When the water is heated off, the

new substance is known as an anhydrate.

The words are red because they are vocab words!!!!

Which is the hydrate and the anhydrate?

Hydrates

The reaction must be performed in a crucible on a clay triangle on a ring stand.

• Clean and heat crucibles before use

• Careful they get really hot!

• Always reheat your sample to ensure all water has left the hydrate

• Record all measurements!

Hydrates

Once the hydrate is heated, you can calculate the moles of water that evaporated, and figure out the formula. The steps are as follows:1. Calculate the mass of hydrate,

anhydrate and water by subtraction.2. Find the moles of anhydrate and water

separately.3. Find the mole ratio of anhydrate to

water (1 to ?).Hint: write this down so you have it for the lab!

Hydrate Example:

Data: Mass of crucible: 28.00 g Mass of crucible and hydrate: 29.60 g Mass of crucible and anhydrate: 29.24 g

Calculate the hydrate formula and the percent water if the anhydrate is CuSO4.

Answer

29.60-28.00 = 1.60 g of hydrate 29.24-28.00 = 1.24 g of anhydrate 1.60-1.24 = 0.36 g of water 0.36/1.60 * 100 = 23% water

0.36/18.016 = 0.1998 moles of water

1.24/159.6 = 0.007769moles of anhydrate

CuSO4.3H2O

Check Your Understanding

Can you explain what a hydrate and an anhydrate are and how to calculate the amount of water in it?

You must be able to…

• Explain the hydrate lab set up in detail.• Calculate the moles of water in a

hydrated sample.

Video AP 1.9Molarity and Dilutions

Objectives

You should already be able to…• Calculate the molarity of a solution to determine

if it is relatively concentrated or dilute.• Use the dilutions formula to calculate the

concentration of a solution after water has been added.

By the end of this video you should be able to….• Use the dilutions formula to calculate the

concentration of a solution before or after water has been added.

Concentration Concentrated

solutions contain large amounts of solutes dissolved in the solvent.

Dilute solutions contain small amounts of solutes dissolved in solvent.

Which is concentrated?

Molarity

The amount of solute (dissolvable substance) in a solvent (does the dissolving like water) is known as concentration and can be represented many ways.

Molarity is the moles of solute per liter of solution. Find this on your reference tables!

The units for molarity are mol/L or mol.L-1 or just M.

If you don’t know what this means, maybe write a note to ask me in class…

mol of soluteL of solutionM =

Molarity (M)

Because volume is temperature dependent, molarity can change with temperature.

Examples can be done without a calculator!

1. If 3.0 moles of NaCl are dissolved in 6.0L of water, what is the molarity?

2. If 29 grams of NaCl are dissolved in one liter of water, what is the molarity?

3moles/6L =

29g/58g = 0.50moles

0.5moles/1L =

0.50M

0.50M

Examples may need a calculator or can be estimated

3. If 100. grams of KF are dissolved in 300.ml of water, what is the molarity?

4. Calculate the volume needed to create a 2.0M solution with 3.5 moles of Li2O.

100g/58.1g =1.72mol

X = 1.8L

1.72mol/.300L =

2.0M=3.5mol/x

5.70M

THINK

If you have 3.12g of HCl and 250mL of H2O, what is the molarity of the acid? If 250mL of H2O is added again, how does the molarity change?

M decreases by half

Check Your Understanding

Can you calculate the concentration of a solution?

THINK

If the glass of iced tea I made is too strong for you, what could you do to make it weaker?

This process is known as dilution. It is very important in chemistry because we often purchase large quantities of highly concentrated chemicals. In order to use the chemicals in class, for labs, teachers must dilute the chemicals so they aren’t as dangerous.

Dilutions

A dilution is a procedure for preparing a less concentrated solution from a more concentrated (or stock) solution. Use the formula: M1V1 = M2V2

Calculations

1. 240mL of a 3.5M solution is diluted to 300 mL of solution. Calculate the new molarity.

2. 7.8L of 2.0M solution is diluted by the addition of 10.0L of water. Calculate the new molarity.

2.8M

0.88M

3.5(240) = x(300)

2.0(7.8) = x(17.8)

Calculations

3. What volume of 9.3M H2SO4 is needed to obtain 450mL of 2.0M H2SO4?

4. What always happens to the molarity when water is added?What would you have to do in order to have the molarity increase?

97mL

M decreases

Increase moles or evaporate water

9.3(x) = 2.0(450)

Check Your Understanding

Can you calculate the concentration of a diluted solution?

You must be able to…

• Use the dilutions formula to calculate the concentration of a solution before or after water has been added.

Video AP 1.10Harder Examples of Dilutions

Objectives

You should already be able to…• Calculate the molarity of a solution to determine

if it is relatively concentrated or dilute.• Use the dilutions formula to calculate the

concentration of a solution after water has been added.

By the end of this video you should be able to….• Use the dilutions formula to calculate the

concentration of a solution before or after water has been added in harder AP problems

Dilutions

1. A lab calls for 100mL of 3.oM HCl per pair of students. The stock room has a large amount of 12.0M HCl and 1.0M HCl. Which acid will you use to set up the lab?

2. If the classroom has 26 students, explain how to create enough 3.0M acid for the lab. Use calculations to support your answer.

Limiters with Molarity

2HCl + Ca(OH)2 2H2O + CaCl2 If 250mL of 5M HCl is reacted with

300mL of 4.5M Ca(OH)2 according to the equation above, what is the limiting reactant and how many moles of CaCl2 are produced when the product is evaporated to dryness?

What you need to say…

If a question asks you how to dilute a solution make sure you mention… Measure the amount of water needed using a pipet (it

is pretty accurate and if not use the cylinder). Place into the volumetric flask (most accurate!!!)

Start with the highest molarity of solute: Measure the concentrated solution with the pipet (or cylinder).

Always add the acid to the water (the acid is more dense and will sink to the bottom resulting in less splashing and can be added slowly to reduced the amount of heat generated at a time).

Add small drops of water using a dropper until the solutions meniscus reaches the mark on the flask.

Write this!

Check Your Understanding

Can you explain how to dilute a solution?

You must be able to…

• Use the dilutions formula to calculate the concentration of a solution before or after water has been added in harder AP problems

Video AP 1.11Harder Examples of AP Stoichiometry

Objectives

You should already be able to…• Calculate moles, mass, volume, number of

particles, percent yield, percent composition, and concentration.

• Determine empirical and molecular formulas.• Balance and identify the type of reaction.By the end of this video you should be able to….• Feel confident about AP Stoichiometry

problems

Answer the following questions about BeC2O4(s) and its hydrate.

(a) Calculate the mass percent of carbon in the hy drated form of the solid that has the formula BeC2O4•3H2O.

(b) When heated to 220.°C, BeC2O4•3H2O(s) dehy drates completely as represented below. If 3.21 g of BeC2O4•3H2O(s) is heated to 220.°C calculate the mass of BeC2O4(s) formed and the moles of water formed.

BeC2O4•3H2O(s) BeC2O4(s) + 3 H2O(g)

The molecular formula of a hydrocarbon is to be determined by analyzing its combustion products and investigating its properties.

(a) The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO2 at standard conditions. What is the empirical formula of the hydrocarbon?

(b) Calculate the mass in grams of O2 required for the complete combustion of the sample of the hydrocarbon described in (a).

You must be able to…

• Feel confident about AP Stoichiometry problems

• Or review your brain off and come to extra help with specific questions!