76
Unit 4: The Mole Concept and Stoichiometry Section 1: Percent Composition and Molar Mass Section 2: Empirical Formulas Section 3: Dimensional Analysis Section 4: The Mole Concept Section 5: Avogadro’s Number Section 6: Mole Ratios & Theoretical Yield Section 7: Limiting Reactants Section 8: Percent Yield Section 9: “Other” Stoichiometry Problems

Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Embed Size (px)

Citation preview

Page 1: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Unit 4: The Mole Concept and Stoichiometry

Section 1: Percent Composition and Molar Mass

Section 2: Empirical Formulas

Section 3: Dimensional Analysis

Section 4: The Mole Concept

Section 5: Avogadro’s Number

Section 6: Mole Ratios & Theoretical Yield

Section 7: Limiting Reactants

Section 8: Percent Yield

Section 9: “Other” Stoichiometry Problems

Page 2: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Unit Synapsis

In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of mass. We also explored molecules and Ionic compounds and how these substances are often represented using a chemical formula.

In this Unit we will further explore the law of conservation of mass and chemical reactions by representing them mathematically.

We will start slowly by looking at the percentage that each element contributes to a substance. After that we will look at reducing molecular formulas.

Next, we will look at one of the fundamental concepts in chemistry; the mole. The mole is the measuring cup of nature, it’s how we determine proportionality in chemical reactions.

The proportionality of chemical reactions and how the mole is used to represent those proportions is the focus of Section 6,7,8, & 9.

Page 3: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 1: Percent Composition & Molar MassThis part of the Unit is covered on pages 341-342 in your textbook

Page 4: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 1: Percent Composition & Molar Mass / Objectives

“After this lesson I can…

• …calculate the molar mass of a given substance.”

• …Determine the percent composition for a given substance when provided it’s chemical formula.”

Page 5: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Percent Composition & Molar Mass

• The percent composition of a molecule or compound is the percentage that each element in the molecule or compound contributes to the molar mass.

• The molar mass is the sum total of all the average atomic masses for all the atoms in a given substance.

• This is simple and rather incorrect definition for molar mass but it works for now. Later on we will look at the more accurate and more important definition.

• Example #1: C6H12O6

• C: 6 × 12.01 = 72.06

• H: 12 × 1.01 = 12.12

• O: 6 × 16.00 = 96

• (72.06 + 12.12 + 96) = 180.18Molar mass of C6H12O6

• Example #2: Na3PO4

• Na: 3 × 22.99 = 68.97

• P: 1 × 30.97 = 30.97

• O: 4 × 16.00 = 64.00

• (68.97 + 30.97 + 64.00) = 163.94molar mass of Na3PO4

Page 6: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Molar Mass

1) Lithium Nitrate: LiNO3

2) Magnesium Fluoride: MgF2

3) Dinitrogen Tetraoxide: N2O4

4) Iron(III) Carbonate: Fe2(CO3)3

5) Caffeine: C8H10N4O2

Directions: Calculate the molar mass for the following molecules and compounds and do it with a smile!

Page 7: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Percent Composition & Molar Mass

• To find the percent composition of a given substance you just take each elements contribution and divide it by the molar mass. You have to convert the decimal to a percent of coarse.

• Example #1: C6H12O6

• C: 6 × 12.01 = 72.06 / 180.18 = .3999 or 39.99% Carbon

• H: 12 × 1.01 = 12.12 / 180.18 = .0673 or 6.73% Hydrogen

• O: 6 × 16.00 = 96 / 180.18 = .5328 or 53.28% Oxygen

• (72.06 + 12.12 + 96) = 180.18 Molar mass of C6H12O6

• Example #2: Na3PO4

• Na: 3 × 22.99 = 68.97 / 163.94 = .4207 or 42.07% Sodium

• P: 1 × 30.97 = 30.97 / 163.94 = .1889 or 18.89% Phosphorus

• O: 4 × 16.00 = 64.00 / 163.94 = .3904 or 29.04% Oxygen

• (68.97 + 30.97 + 64.00) = 163.94 molar mass of Na3PO4

Page 8: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Percent Composition

1) Lithium Nitrate: LiNO3

2) Magnesium Fluoride: MgF2

3) Dinitrogen Tetraoxide: N2O4

4) Iron(III) Carbonate: Fe2(CO3)3

5) Caffeine: C8H10N4O2

Directions: Determine the Percent Composition for the following molecules and compounds.

Page 9: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 1: Additional Resources

• Tyler Dewitt’s Video: “Percent Composition by Mass”

Page 10: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 2: Empirical FormulasThis part of the Unit is covered on pages 344-350 in your textbook

Page 11: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 2: Empirical Formulas / Objectives

“After this lesson I can…

• …determine the empirical formula of a substance when provided it’s molecular formula.”

• …calculate the empirical formula of a substance when given it’s percent composition.”

• …calculate the empirical formula of a substance when given the masses for the elements in that substance.”

• …determine the molecular formula for an unknown substance when given it’s molar mass and either masses of elements or percent’ composition.”

Page 12: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Empirical Formula Defined

• The chemical formula of a substance is the number and kinds of atoms that make up a molecule or ionic compound.

• Molecular formula is the same thing but it only applies to molecules and not ionic compounds.

• Empirical formula is a math manipulation of molecular formula. It is essentially the molecular formula reduced to it’s lowest whole numbered ratios.

• Empirical formulas don’t apply to ionic compounds because they are already reduced to their lowest whole number ratios.

• Examples

• Molecular formula: C5H10 has the empirical formula CH2

• Molecular formula: C4H8 has the empirical formula CH2

• Molecular formula: C3N12 has the empirical formula CN4

• Molecular formula C6H12O6 has the empirical formula CH2O

• Not all molecules can be reduced to empirical formulas though. Sometimes a molecular formula is not reducible and so the molecular formula is the empirical formula.

• Examples of those include: P3N5 & C5H12

Page 13: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Empirical Formulas

1) N2O4

2) C6H12

3) C2H6O

4) C8H10N4O2

5) P2F4

6) P4O10

7) C6H18O3

8) S5N6

Directions: Determine the empirical formula for the following molecules. If the molecule’s chemical formula cannot be reduced write “cannot be reduced”

Page 14: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Why Empirical Formulas Matter (way back when)

• Getting the molecular formula from the empirical formula, or going in the reverse direction is not possible unless you have the molar mass.

• Why? Well suppose the empirical formula is CH2. There are many different molecular formulas for CH2 such as; C2H4, C3H6, C4H8, and the list goes on. Without any other information there is no way to know what molecule the empirical formula CH2 is representing.

• So it’s impossible to determine the molecular formula if all you have is the empirical formula.

• However, it is possible to determine molecular formula from empirical formula if you have the molar mass as well.

• In fact, 80 years ago that’s how chemists and researches determined the molecular formulas for an unknown substances. One instrument would get the data for the empirical formula and another would get the molar mass. Next, you could do some math with these two pieces of information to determine the molecular formula.

• Nowadays, there are instruments than can identify molecular formula more directly. Still, the process for determining molecular formula via molar mass and empirical formula is still taught in high school chemistry (lucky you).

Page 15: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Determining Molecular Formula

“A substance is analyzed and found to have the empirical formula CH2. If the molar mass is found to be approximately 84 g/mol, what is the molecular formula?”

“A substance is analyzed and found to have the empirical formula NBr2. If the molar mass is found to be approximately 347.6 g/mol, what is the molecular formula?”

Page 16: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Determining Molecular Formula

“A substance is analyzed and found to have the empirical formula PF2. If the molar mass is found to be approximately 137.9 g/mol, what is the molecular formula?”

“A substance is analyzed and found to have the CH2O. If the molar mass is found to be approximately 60 g/mol, what is the molecular formula?”

Page 17: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Empirical Formulas from Percent Composition or Mass

• Many analytical instruments widely available today can determine the percent composition of an unknown substance with relative ease, and from there more fun math can be used to get the empirical formula.

• Percent composition can than be used to determine an unknown substance’s empirical formula using the following steps:

1) Divide each elements percentage by it’s average atomic mass.

2) Take the lowest number from step 2 and divide the other numbers by it.

3) If the answers from step two are reasonably close to whole numbers, round them to whole numbers. Those are the subscripts for each element. If they are not, multiply to get whole numbers.

• These steps will work if you are given the grams of each element in the formula as well. Just divide the grams of each element by the average atomic mass instead of the percentages.

Page 18: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Empirical Formula from Percent Composition

“Find the empirical formula for a compound which contains 32.8% chromium and 67.2% chlorine?”

“What is the empirical formula for a compound that contains 26.1% carbon, 4.3% hydrogen and 69.6 % oxygen?”

Page 19: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Empirical Formula from Mass

“If a sample of ethyl butyrate is known to contain 0.62069 g of carbon, 0.103448 g of hydrogen and 0.275862 g of oxygen, what is the empirical formula for ethyl

butyrate?”

“What is the empirical formula for a compound that contains 5.28g of Tin and 3.37g of Fluorine?"

Page 20: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Empirical Formula Concept Map

Page 21: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 2: Additional Resources

• Tyler Dewitt’s Video: “Empirical Formula and Molecular Formula Introduction”

• Tyler Dewitt’s Video: “Calculating Molecular Formula from Empirical Formula: Part 2”

• The Organic Chemistry Tutor’s Video: “Empirical Formula Given Grams and & Mass Percent Composition, Molecular Formula Determination”

Page 22: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 3: Dimensional AnalysisThis part of the Unit is covered on pages 44-46 in your textbook

Page 23: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 3: Dimensional Analysis / Objectives

“After this lesson I can…

• …identify the conversion factor(s) in a given problem or find the conversion factor(s) in reference materials.”

• …use dimensional analysis to solve single and multi-step problems (particularly mole problems)”

Page 24: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Dimensional Analysis

• Many of you have already used dimensional analysis in the past to convert between units. Some people call it “fence posting”

• If you look up dimensional analysis on Wikipedia you will get a very accurate but confusing definition. The best way to define dimensional analysis is that it is a technique for converting between units or solving problems.

• This technique is especially useful when more units have to be converted in successive steps, like when you have to go to pounds to kilograms and then to grams.

• Dimensional Analysis is about canceling out units until you arrive at your desired unit (the answer).

• It is also very useful for things like physics and engineering and those of you who go on to take those classes will be re-introduced to the dimensional analysis technique.

Page 25: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Steps for using Dimensional Analysis

1) Identify the conversion factors you need to use and write them off

to the side using an equal sign.

2) Write down the quantity you are starting with.

3) Next to the quantity you are starting with, draw a number of “fence

posts” equal to the number of conversion factors you are using

(example: 3 conversion factors needs 3 fence posts, 5 conversion

factors needs 5 fence posts)

4) Find the conversion factor you need to cancel out the unit that you

have.

5) Write that conversion factor as a fraction in the first fence post. Put

the canceling unit in the denominator and the other unit in the

numerator (Remember: “Whatever comes up, must come down”)

6) Repeat steps 4 and 5 with subsequent conversion factors until you

have arrived at the unit the question is asking for.

Page 26: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Dimensional Analysis

“The guy at the game store will give you 1 PS4 game for 3 xbox games. How many PS4 games can you get with 15 xbox games?”

“6 bags of chips can be traded for 4 Gatorades. 24 Gatorades can be traded for 2 pizzas. How many pizzas can you get with 138 bags of chips?”

Page 27: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Dimensional Analysis

“12 apples gets you 35 bananas at the market. 5 bananas gets you 2 fruit roll ups. 3 fruit roll ups can be traded for 1 can of coke. How many cans of coke can you get

with 100 apples?”

“Game tokens are 100 for a dollar. The crane game takes 25 tokens. If you play the crane game 2 times you win 5 pieces of candy. It takes 12 pieces of candy to make

you sick for 2 hours. How many hours are you going to be sick if you have 9 dollars?”

Page 28: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 3: Additional Resources

• Tyler Dewitt’s Video: “Understanding Conversion Factors”

• Tyler Dewitt’s Video: “Converting Units using conversion Factors”

• Tyler Dewitt’s Video: ”Converting Units using Multiple Conversion Factors”

Page 29: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 4: The Mole ConceptThis part of the Unit is covered on pages 325-340 in your textbook

Page 30: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 4: The Mole Concept / Objectives

“After this lesson I can…

• …convert between grams and moles.”

Page 31: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Moles

• Moles measures the amount of something. A lot of you are used to thinking of grams, pounds, or volume as how “much” of something you have but really how “much” of something you have is measured in moles.

• It’s moles because moles measures the total number of particles.

• Volume measures the space an object takes up.

• Mass measures the object’s resistance to acceleration.

• Moles measures the amount of an object.

• In 1 mole of a thing, there are 602,000,000,000,000,000,000,000 particles of that thing.

• Particles could be molecules if you are dealing with molecules, it could be atoms if you are dealing with a pure element like a block of iron, or it could be “unit cells” of an ionic compound (because ionic compounds have a crystal lattice.”

• Luckily converting between the grams and moles of substance is simple; molar mass is your conversion factor! Molar mass is best defined as how many grams there are in one mole.

Page 32: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Mole Conversions

“How many moles of Water in 111 grams?”

“How many moles of C6H12O6 in 250 grams of C6H12O6?”

Page 33: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Mole Conversions

“Suppose you have 4.3 moles H2O, How many grams is this?”

“Calculate the number grams in .044 moles of CrPO4.”

Page 34: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 4: Additional Resources

• Tyler Dewitt’s Video: “Converting Between Grams and Moles:”

• Tyler Dewitt’s Video: “Converting Between Grams and Moles: Part 2”

Page 35: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 5: Avogadro’s NumberThis part of the Unit is covered on pages 320-324 in your textbook

Page 36: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 5: Avogadro’s Number / Objectives

“After this lesson I can…

• …convert between moles and number of particles.”

• …solve multi-step problems involving Avogadro’s number”

Page 37: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Avogadro’s Number

• As previously stated, in one mole of a substance there are 6.02 × 1023

particles of that substance.

• Particles could mean molecules, “formula units” in the case of ionic compounds, or atoms in the case of pure substances.

• The number 6.02 × 1023 is known as Avogadro’s number. It’s named in honor of Ameado Avogadro who did early experiments with gases.

• Usually when doing chemistry you just leave things in moles, but if you want to know how many particles there are in anything, get it into moles and then use the conversion factor:

1 mole = 6.02 × 1023 particles.

Page 38: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

VIDEOS!

• TedEd Video: “How Big is a Mole”

Page 39: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Avogadro’s Problems

“How many molecules of Water in 11.45 moles?”

“How many Aluminum atoms in a piece of Aluminum foil that contains 15.1 moles?”

Page 40: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Avogadro’s Problems

“How many molecules of Carbon Dioxide in .00045 moles?”

““How many moles of NO2 in 2.22 × 1025 molecules?”

Page 41: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Avogadro’s Problems

“Suppose a mineral has 2.22 × 1021 units of CuCO3, How many moles of Copper Carbonate in the mineral?”

“How many moles of Dinitrogen Tetroxide in 2.22 × 1025 molecules?”

Page 42: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Avogadro’s Problems

“How many molecules of Water in 250 grams?”

“How many Aluminum atoms in piece of Aluminum foil that has a mass of 155 grams?”

Page 43: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Avogadro’s Problems

““Suppose a mineral has 2.22 × 1027 units of CuCO3, How many grams of Copper Carbonate in the mineral?”

“How many Hydrogen atoms in in 155 grams of CH4?”

Page 44: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 5: Additional Resources

• Tyler Dewitt’s Video: “Converting Between Moles, Atoms and Molecules”

• Tyler Dewitt’s Video: “Converting Between Moles, Atoms, and Molecules Part 2”

Page 45: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 6: Mole Ratios and Theoretical YieldThis part of the Unit is covered on pages 368-378 in your textbook

Page 46: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 6: Mole Ratios & Theoretical Yield / Objectives

“After this lesson I can…

• …use the mole ratios in a balanced chemical equation to calculatetheoretical yield when given the grams or moles of reactant.”

Page 47: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Mole Ratios

• The mole ratio is taken from a balanced chemical equation and is simply the ratio of coefficients between reactants and products.

• Mole ratios are really conversion factors.

• Example: 2 C2H6 + 7 O2 4 CO2 + 6 H2O

• Here are some of the ratios in this problem…

• 2 moles of C2H6 = 4 moles of CO2

• 2 moles of C2H6 = 6 moles of H2O

• 7 moles of O2 = 4 moles of CO2

• 7 moles of O2 = 6 moles of H2O

• These ratios allow us to calculate the amount of products that will form from a given amount of reactants.

• For example if we have 2 moles of C2H6 than 4 moles of CO2 will be produced (assuming there is plenty of O2 for it to react with).

• If we happen to have 4 moles of C2H6 then 8 moles of CO2 will be produced (again, assuming there is enough of O2 for it to react with).

Page 48: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Theoretical Yield

• Theoretical Yield is the maximum amount of product that can form given the amount of reactant that you are starting with. It is calculated using the mole ratio to convert the amount of reactant to product.

• Sometimes problems ask for theoretical yields in moles, sometimes in grams, and sometimes in liters of a gas. it doesn’t have real world applications, but a problem could even ask for the theoretical yield in the number of particles.

• Theoretical yield is ALWAYS found in moles first. Therefore, if a problem asks for the theoretical yield in something else, you will have to find it in moles first and then convert it to that other thing later on.

• Likewise, you need to have your reactant in moles to find theoretical yield. So if your reactant is given to you in something else, you will have to convert it to moles first.

• These relationships are summarized in a diagram on the next page. You can think of the diagram as a flow chart. We are only worried about conversion involving mass and Avogadro’s number in this unit. Conversion involving Volume of a gas and Liters of solution will be done in Units 5 & 6 respectively

Page 49: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Stoichiometry Concept Map

Page 50: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Theoretical Yield

Suppose 1.9 moles of C9H20 reacts with an excess of Oxygen gas. What is the theoretical yield of CO2 in moles given the reaction:

C9H20(l) + 14 O2(g) 9 CO2(g) + 10 H2O(g)

“If 5.5 moles of N2O5 breaks down, how many moles of NO2 will be produced given the reaction:

2 N2O5(g) 4 NO2(g) + O2(g)

Page 51: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Theoretical Yield

Suppose 55 grams of C9H20 reacts with an excess of Oxygen gas. What is the theoretical yield of CO2 in moles given the reaction:

C9H20(l) + 14 O2(g) 9 CO2(g) + 10 H2O(g)

“If 3.7 moles of N2O5 breaks down, how many grams of NO2 will be produced given the reaction:

2 N2O5(g) 4 NO2(g) + O2(g)

Page 52: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Theoretical Yield

Suppose 112 grams of C9H20 reacts with an excess of Oxygen gas. What is the theoretical yield of CO2 in grams given the reaction below:

C9H20(l) + 14 O2(g) 9 CO2(g) + 10 H2O(g)

“If 15 grams of N2O5 breaks down, how many grams of NO2 will be produced given the reaction:

2 N2O5(g) 4 NO2(g) + O2(g)

Page 53: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 6: Additional Resources

• Tyler Dewitt’s Video: “Mole Ratio Practice Problems”

Page 54: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 7: Limiting ReactantsThis part of the Unit is covered on pages 379-384 in your textbook

Page 55: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 7: Limiting Reactants / Objectives

“After this lesson I can…

• …Identify the limiting reactant in theoretical yield problems and then solve those problems.

Page 56: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Limiting Reactant

• Notice in Section 6 we really only looked at how to calculate the theoretical yield if we know the amount of 1 reactant.

• Previous problems often stated that there was an “excess” of the other reactant, which means there is more of it than you need.

• What if we are given amounts for two or more reactants? Which one will we used to determine the theoretical yield?

• While we will have to do the same process we have been doing for both reactants, the theoretical yield is ultimately determined by whichever reactant gives us the lesser amount of product. This reactant is called the limiting reactant, because it limits the amount of product that can be made. The other reactant, the non-limiting reactant, has a certain amount that is left over.

Page 57: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Limiting Reactant

“Suppose 2.7 moles of C9H20 reacts with 4.6 moles Oxygen gas. What the theoretical yield of CO2 in grams given the reaction:

C9H20(l) + 14 O2(g) 9 CO2(g) + 10 H2O(g)? “

Page 58: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Limiting Reactant

“If you have 100 grams of Copper react with 100 grams of HNO3, what is the theoretical yield of Copper Nitrate in grams? What is the limiting reactant?”

3 Cu + 8 HNO3 3 Cu(NO3)2 + 2 NO + 4 H2O

Page 59: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Limiting Reactant

“What mass of Magnesium Iodide will results from the reaction of 500 grams of HI with 300 grams of Magnesium Oxide? “

MgO(s) + 2 HI(s) MgI2(aq) + H2O(l)

Page 60: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

A Quick Note on Making Assumptions

• If theoretical yield problem that only gives you the amount for one of the reactants, just assume that reactant is the limiting reactant and that you have an excess of the other reactant(s).

Examples:

• “What the theoretical yield of NO2 in moles if 100 grams of Sulfur reacts? S + 6 HNO3H2SO4 + 6 NO2 + 2 H2O”

• Assume there is an excess of HNO3

• “How many grams of NaHCO3 can be made if 45 grams of Na2O is used in the following reaction: Na2O + 2 CO2 + H2O 2 NaHCO3”

• Assume there is an excess of CO2 and H2O.

Page 61: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 7: Additional Resources

• Tyler Dewitt’s Video: “Introduction to Limiting reactant and excess reactant”

Page 62: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 8: Percent YieldThis part of the Unit is covered on pages 385-388 in your textbook

Page 63: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 8: Percent Yield / Objectives

“After this lesson I can…

• …Distinguish actual yield, theoretical yield, and percent yield”

• …calculate percent yield after solving a theoretical yield problem (including those that involve limiting reactants), if a story problem provides actual yield or it’s obtained in a lab.”

Page 64: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Actual Yield

• Remember that Theoretical yield is the maximum possible amount of product you can get from a reaction if you know the amount of reactants you are starting with.

• In the real world, it is virtually impossible to obtain the theoretical yield.

• The Actual yield is what you actually recover after the reaction and any purification steps are over.

• Actual yield is always less than the theoretical yield. There are several reason why this is the case. One common reason is that you lose some of your product along the way (it’s not entirely removed from the reaction flask, it’s not entirely removed from the filter paper, ect…)

• Another reason is that often times the limiting reactant does not completely react.

Page 65: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Percent Yield

• The percent yield is the actual yield divided by the theoretical yield times 100.

• It’s a percentage of the product you recovered.

• In the business world, the higher the percent yield the better because any product not recovered is essentially money lost.

• Pharmaceutical companies for example will try and fine tune their manufacturing process so they have very high percent yields for the drugs they make.

• It is impossible for percent yield to be greater than 100%, if this happens in the real world (like in a lab you do), there are only two possibilities:

• You made an error when calculating theoretical yield

• The product you recovered is impure; it contains other substances that are adding mass like water or left over reactant. Keep in mind water can be present even in substances that appear dry.

Page 66: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Percent Yield

Suppose 90 grams of Na2O reacts with an excess of Carbon Dioxide and Water according to the reaction below. If 189 grams of NaHCO3 are recovered, what is the

percent yield?

Na2O + 2 CO2 + H2O 2 NaHCO3

Page 67: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Percent Yield

“Suppose 2.6 grams of Na3PO4 reacts with 3.5 grams of CaCl2 according to the reaction below. If .55 grams of Ca3(PO4)2 are recovered, what is the percent yield?”

2 Na3PO4 + 3 CaCl2 Ca3(PO4)2 + 6 NaCl

Page 68: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 8: Additional Resources

• The Organic Chemistry Tutor Video: “How to Calculate Theoretical Yield and Percent Yield”

Page 69: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 9: “Other” Types of Stoichiometry ProblemsThis part of the Unit is covered on pages 368-384 in your textbook

Page 70: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 9: “Other” Types of Stoichiometry Problems / Objectives

“After this lesson I can…

• …Solve stoichiometry problems that ask for amounts of reactants instead of products.

• …Solve stoichiometry problems that ask for how much of a reactant you need to completely react with another reactant.

• …solve stoichiometry problems that ask for how much a non-limiting reactant is left over.

Page 71: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

“Other” Stoichiometry Problems

• You might encounter Stoichiometry problems that ask for something besides theoretical yield.

• One type of problem might instead ask for you to calculate the amount of reactant needed to form a desired amount of product.

• Another type of problem might ask how much of a reactant you might need to “completely react” with another reactant. In these types of problems it is as if the problem is asking you to insure there is not a limiting reactant; that both reactants are used up completely.

• Once again you will rely on mole ratios to solve these problems. If you are using Dimensional Analysis these problems should not be too difficult. The math is done exactly the same, your just solving for something else besides the product.

Page 72: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: “Other” Stoichiometry Problems

How many grams of S8 do you need to completely react with 22 grams of Cl2?

S8(s) + 4 Cl2(g) 4 S2Cl2(l)

Page 73: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: “Other” Stoichiometry Problems

“If you want to synthesis 25 grams of Ammonia, how many grams of H2 and how many grams of N2 will you need?”

N2(g) + 3 H2(g) 2 NH3(g)

Page 74: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Percent Yield

“Suppose 16 grams of Na3PO4 reacts with 44 grams of CaCl2 according to the reaction below. What is the limiting reactant, what is the theoretical yield of Calcium

Phosphate, and how much of the non-limiting reactant will be left over?

2 Na3PO4 + 3 CaCl2 Ca3(PO4)2 + 6 NaCl

Page 75: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Practice Problems: Percent Yield

“Suppose 2.6 grams of Na3PO4 reacts with 3.5 grams of CaCl2 according to the reaction below. If .55 grams of Ca3(PO4)2 are recovered, what is the percent yield?”

2 Na3PO4 + 3 CaCl2 Ca3(PO4)2 + 6 NaCl

Page 76: Unit 4: The Mole Concept and Stoichiometry · Unit Synapsis In the previous unit we explored chemical reactions and how they must be balanced because of the law of conservation of

Section 7,8,9: Additional Resources

• Tyler Dewitt’s Video: “Limiting Reactant Practice Problem”

• Tyler Dewitt’s Video: “Limiting Reactant Practice Problem (Advanced)”

• The Organic Chemistry Tutor Video: “Theoretical, Actual, Percent Yield & Error - Limiting Reagent and Excess Reactant That Remains”

• The Organic Chemistry Tutor Video: “Excess Reactant Stoichiometry, Left Over, Limiting Reactant, Theoretical & Percent Yield Practice”