1. Chapter 9 Calculations from Chemical Equations (Stoichiometry)
Objectives:
The Mole-Ratio Method
Mole-Mole Calculations
Mole-Mass Calculations
Mass-Mass Calculations
Limiting-Reactant and Percent Yield Calculations

2. A Short Review Molar Mass Molecules and Moles Balanced Equations
Sum of the atomic masses of all atoms in a molecule
Molecules and Moles
A molecule is the smallest unit of a substance
A mole is Avogadro’s number (6.022 x 1023) molecules of that substance
Balanced Equations
Equations must be balanced
Number in front of a formula in a balanced chemical equation represent the number of moles of that substance

3. The Mole-Ratio Method Stoichiometry Mole ratio
quantitative relationships among reactants and products
Mole ratio
Ratio between the number of moles of any two species involved in a chemical reaction
2H2 + O2  2H2O
Six mole ratios can be written:
2 mol H2
1 mol O2
2 mol H2
2 mol H2O
1 mol O2
2 mol H2
1 mol O2
2 mol H2O
2 mol H2O
2 mol H2
2 mol H2O
1 mol O2

4. The Mole-Ratio Method = 8.0 mol H2O
Use the mole ratio to convert number of moles of one substance to number of moles of another substance
How many moles of H2O can be obtained from 4.0 moles of O2?
4.0 mol O mol H2O
1 mol O2
= 8.0 mol H2O

5. The Mole-Ratio Method Three basic steps:
Convert the quantity of starting substance to moles (if it is not given in moles)
Convert the moles of starting substance to moles of desired substance
Convert the moles of desired substance to the units specified in the problem
Grams of B
Grams of A
Moles of A
Moles of B
Atoms or Molecules
hof A
Atoms or Molecules
of A

6. Moles Of Given Moles Of Unknown
Quantity of Given
(mass, atoms, molecules)
Quantity of Unknown
(mass, atoms, molecules)
Convert to Moles
Convert to desired units
Moles Of
Given
Find molar ratio
Moles Of
Unknown

7. Mole-Mole Calculations
Quantity of given substance in moles
Quantity of desired substance requested in moles
How many moles of carbon dioxide will be produced by the complete reaction of 2.0 mol of glucose (C6H12O6) according to the following reaction?
C6H12O6 + 6O2  6CO2 + 6H2O
2.0 mol C6H12O mol CO2
1 mol C6H12O6
= 12 mol CO2

8. Mole-Mole Calculations
N2H4 + 2H2O2  N2 + 4H2O
If you have 3 moles of N2H4 how many moles of N2 will you produce?
3 mol N2H4 1 mol N =
1 mol N2H4
How many molecules of H2O2 are needed to produce 3 moles of H2O?
3 mol H2O 2 mol H2O X 1023 molecules
4 mol H2O 1 mol H2O2
3 mol N2
= 9 x 1023 molecules H2O2

9. Mole-Mass Calculations
Calculate the mass of one substance when given moles of another
>> OR <<
Calculate the moles of one substance when given mass of another
Mass of substance 1 to moles of substance 1 to moles of substance 2 (using mole ratio)

10. Mole-Mass Calculations
2KClO3  2KCl + 3O2
How many grams of potassium chlorate are needed to produce 5 moles of oxygen?
How many moles of potassium chloride can be produced from g of potassium chlorate?
Do these problems on the board!
5 moles O2 | 2 mol KClO3 | g KClO = g KClO3
| 3 mol O | 1 mol KClO3
100.0 g KClO3 | 1 mol KClO3 | 2 mol KCl = mol KCl
| KClO3 | 2 mol KClO3

11. Mass-Mass Calculations
Calculate the mass of one substance when given the mass of another
Mass of substance 1  moles of substance 1  moles of substance 2  mass of substance 2

12. Mass-Mass Calculations
2AgNO3 + H2S  Ag2S + 2HNO3
How many grams of silver nitrate are required to produce grams of silver sulfide?
How many grams of nitric acid will be produced if 325 grams of H2S is used?
250 g Ag2S | 1 mol Ag2S | 2 mol AgNO3 | g AgNO3 = g AgNO3
| g Ag2S | 1 mol Ag2S | 1 mol AgNO3
325 g H2S | 1 mol H2S | 2 mol HNO3 | g HNO3 = g = 1200 g HNO3
| g H2S | 1 mol H2S | 1 mol HNO3

13. Mass-Mass Calculations
What mass of water is produced by the complete combustion of g of butane (C4H10)?
Write reaction
Balance
Solve problem
2C4H O2  8CO2 + 10H2O
225.0 g C4H10 | 1 mol C4H10 | 10 mol H2O | g H2O = g H2O
| g C4H10 | 2 mol C4H10 | 1 mol H2O

14. Part One Homework Paired Exercises # 3-19 odd
Additional Exercises #33 & 37
Enjoy your Thanksgiving!!!
See you Tuesday, December 2nd
Be prepared for a QUIZ over this information! 

15. Limiting Reactants Often, quantities of reactants are not perfect
One may be left over
Amount of product formed depends on reactant that is not in excess
Limiting reactant
One batch of chocolate chip cookies requires 4 eggs, 4 cups of flour, and 12 oz. of chocolate chips and produces 45 cookies.
How many batches can be made from 1 dozen eggs, 15 cups of flour, and 36 oz. of chocolate chips?
How many cookies will be produced from 8 eggs, 4 cups of flour, and 24 oz. of chocolate chips?
3 batches (eggs are limiting reactant)
Flour is limiting reactant, only 1 batch, so 45 cookies

16. Limiting Reactants H2 + Cl2  2HCl
How many grams of hydrogen chloride can be produced from g of hydrogen and 50.0 g of chlorine?
Calculate mass-mass for BOTH reactants
The one with the LEAST product is your limiting reactant (and thus your answer)
0.490 g H2 | 1 mol H2 | 2 mol HCl | g HCl = g HCl
| mol H2 | 1 mol H2 | 1 mol HCl
50.0 g Cl2 | 1 mol Cl2 | 2 mol HCl | g HCl = g HCl
| g Cl2 | 1 mol Cl2 | 1 mol HCl
Chlorine is limiting reactant…answer is g HCl

17. Limiting Reactants
If 36.5 g HCl is reacted with 85.6 g of Ba(OH)2 which is the limiting reactant? How much BaCl2 will be produced?
Barium hydroxide is the limiting reactant
104 g BaCl2 will be produced

18. Percent Yield Thus far, quantities represent maximum yield (100%)
Actual yield in lab is not often 100%
Side reactions
Many reactions reversible
Poor lab skills
Expected (theoretical) yield
calculated amount of product that can be obtained
Actual yield
Amount actually obtained

19. Percent Yield Percent yield
Ratio of actual yield to theoretical yield
Actual yield x 100 = percent yield
Expected yield
Determine the percent yield for the reaction between 2.80 g Al(NO3)3 and excess NaOH if g Al(OH)3 is recovered. (your second product is NaNO3
Al(NO3)3 + 3NaOH  Al(OH)3 + 3NaNO3

20. Percent Yield
A student places an iron nail with a mass of 2.32 g into a flask of CuSO4. The nail reacts completely, leaving a quantity of copper metal in the bottom of the flask. The student finds the mass of the recovered copper to be 2.51 g. The equation for this reaction is: Fe + CuSO4  FeSO4 + Cu
What is the expected yield?
What is the percent yield?

21. Percent Yield
When octane (C8H18) is burned in oxygen, carbon dioxide and water are produced. If 320 g of octane are burned and 392 g of water are recovered, what is the percent yield of the experiment?
First, write and balance the equation
Then, answer the question

22. Percent Yield 4Al + 3O2  2Al2O3
If 0.25 mol Al and 0.40 mol O2 are reacted, which is the limiting reactant?
If a student reports a percent yield of 75.5%, how much product (in grams) did she recover?

23. Part Two Homework Questions # 1 & 2 Paired Exercises # 21, 23, 27 & 29
Additional Exercises # 31 & 41
Be prepared for TEST on Thursday
Last EXAM of the quarter…
Remember, EC due December 9th