1. The Math of Chemical Reactions
Unit 8 - Stoichiometry
The Math of Chemical Reactions

2. Recall…
In our last unit, we talked about how to convert between moles & mass and moles & atoms/molecules for one compound.
In this unit, we will be applying these skills to convert between moles and mass of two different compounds.

3. What is Stoichiometry?
The study of the mathematical relationships between the substances in balanced chemical reactions
Allows for calculation of amounts of reactants or products in a chemical reaction
i.e. How much of one thing can you make if you start with a certain amount of another thing?

4. Stoichiometry in Chemistry
2 H2 + O2  2 H2O
Coefficients in a balanced chemical equation indicate the relative amounts of each substance involved in the reaction
One way to describe the ratios involved in the reaction above would be “two molecules of hydrogen gas plus one molecule of oxygen gas yields two molecules of water”

5. Mole Ratios
However, since these are only ratios, using the same equation,
2 H2 + O2  2 H2O
It is equally true that, “two moles of hydrogen plus one mole of oxygen yields two moles of water”
These mole relationships are called “mole ratios”

6. Mole Ratio N2 (g) + 3H2 (g)  2NH3 (g)
Mole ratio – conversion factor which relates moles of one compound to moles of another compound in a balanced equation
Coefficients from balanced equation are used to write mole ratios
N2 (g) + 3H2 (g)  2NH3 (g)

7. Mole Ratios
Example #1: What is the mole to mole ratio of oxygen to water in the following equation:
2 H2 + O2  2 H2O

8. ___H2SO4 + ___ NaOH  ___Na2SO4 + ___H2O
Mole Ratios
Example #2: Balance the equation and then determine the mole ratio of sodium hydroxide to sodium sulfate in the following equation:
___H2SO4 + ___ NaOH  ___Na2SO4 + ___H2O

9. Stoichiometry Flowchart

10. Warm Up: 1. Balance the equation: ___Na + ___Cl2  ___NaCl
2. What is the molar ratio between Na and Cl2?
3. What is the molar ratio between Cl2 and NaCl?

11. Stoichiometric Calculations
Mole to Mole Calculations – converting from moles of one compound to moles of a different compound
Write and balance the equation if not already done
Write the mole ratio between the two mentioned compounds using the coefficients from the balanced equation. This will serve as your conversion factor.
Use dimensional analysis & the mole ratio to convert from moles of compound #1 to moles of compound #2

12. Mole to Mole Calculations
Ex #1: In the equation below, if 0.5 moles of magnesium hydroxide react, how many moles of water would be produced?
2 H3PO Mg(OH)2  Mg3(PO4) H2O

13. Mole to Mole Calculations
Ex #2: In the equation below, if moles of H3PO4 react, how many moles of water would be produced?
2 H3PO Mg(OH)2  Mg3(PO4) H2O

14. Mole Conversions WS – Prob. 1
Aluminum metal and hydrogen chloride react to form aluminum chloride and hydrogen gas.
How many moles of aluminum metal are needed to produce 3.33 moles of aluminum chloride?

15. Mole Conversions WS - Answers
1. a mol Al b mol HCl 2. a mol NaBr b mol Al(OH)3 3. a. 3.5 x 10-4 mol CH4 b. 7.0 x 10-4 mol O2

16. Warm-Up:
Balance the following equation: __N2 + __H2  __NH3 If you have 4.5 moles of hydrogen gas, how many moles of ammonia (NH3) would you produce?

17. General Tips for Stoic. Problems
Always convert the given value to moles as your first step!
Once you get to moles, you can convert to moles of the unknown
Moles of unknown can then be converted to whatever final units the problem asks for (grams, liters, milliliters, molecules, etc.)

18. Mass to Mass Calculations
Convert from grams of given compound to moles of given compound (using molar mass)
Convert from moles of given to moles of unknown (using the mole ratio)
Convert from moles of unknown to grams of unknown (using molar mass)

19. Mass to Mass Calculations
Master Formula for Mass to Mass Calculations

20. Mass to Mass Calculations
Ex #1: In the unbalanced equation below, how many grams of carbon dioxide will be produced by the reaction of 108 grams of C5H12? ___ C5H12 + ___ O2  ___ CO2 + ___ H2O

21. Mass to Mass Calculations
Ex #2: In the balanced equation below, how many grams of C5H12 would be needed to completely react with 11.5 grams of oxygen? C5H O2  5 CO2 + 6 H2O

22. Answers for Mass-Mass WS
15.2 g KCl
A g H B g NH3
A g AgCl B g BaCl2

23. Percent Yield
Theoretical yield – amount of product that should be formed when the limiting reactant (the reactant that runs out first) is completely used up
Actual yield – the actual amount of product produced when the reaction is carried out experimentally
Percent yield =

24. Percent Yield
Example #1: If a student made 1.72 g of NaCl in the lab when they should have made 2.00 g, what is the percent yield?

25. Percent Yield
Example #2: N2 + 3 H2  2 NH3 A student calculates that for the reaction above, 5.0 grams of NH3 should be produced. After carrying out the reaction, the student made 4.5 g of NH3. What is the percent yield of the reaction?

26. Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate? (b) If a student did this reaction and produced 2.13 g of zinc phosphate, what is their % yield?

27. Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate?

28. Example #3
___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate? (b) If a student did this reaction and produced 2.13 g of ammonium phosphate, what is their % yield?

29. Answers to Percent Yield WS
1. a g b. 56% 2. a. 8.2 g b. 76% 3. 42%

30. 0.290 mole H2O 1.68 g H2O a. 1.62 g H3PO4 b. 93% (Good)
Answers to HW #5
0.290 mole H2O
1.68 g H2O
a g H3PO4
b. 93% (Good)

31. Limiting Reactant
The reactant that is used up first in a chemical reaction
Determines how much product can be formed by a reaction
The reactant with fewer moles present at the beginning of the reaction

32. Excess Reactant
Reactant present in a quantity that is more than sufficient to react with the limiting reactant
Any reactant that remains after all the limiting reactant is used up
The reactant present with more moles at the beginning of the reaction

33. Limiting Reactant Balloon Demo
What was the limiting reactant for the first balloon? _____________________________
MMbaking soda= g/mol NaHCO3
MMvinegar= g/mol CH3COOH
Moles baking soda:
Moles vinegar:

34. Limiting Reactant Balloon Demo
What was the limiting reactant for the second balloon? _____________________________
MMbaking soda= g/mol NaHCO3
MMvinegar= g/mol CH3COOH
Moles baking soda:
Moles vinegar:

35. Balloon Demo Questions
Why were the third and fourth balloons the same size even though more baking soda was added to the fourth balloon?
Limiting reactant 3rd balloon:
Limiting reactant 4th balloon:

36. Stoichiometry Flowchart