1. II. Stoichiometry in the Real World

2. A. Limiting Reactants Available Ingredients 4 slices of bread
1 jar of peanut butter
1/2 jar of jelly
Limiting Reactant
bread
Excess Reactants
peanut butter and jelly

3. A. Limiting Reactants Limiting Reactant used up in a reaction
determines the amount of product
Excess Reactant
added to ensure that the other reactant is completely used up
The cheaper and more common chemical is used in excess

4. A. Limiting Reactants 1. Write a balanced equation.
2. For each reactant, calculate the amount of product formed.
(this means you will set up 2 stoich problems)
3. Smaller answer indicates:
limiting reactant
amount of product that can be made

5. A. Limiting Reactants Zn + 2HCl  ZnCl2 + H2 79.1 g ? L 2.25 mol
79.1 g of zinc react with 2.25 mol of HCl.
Identify the limiting and excess reactants.
How many liters of hydrogen are formed at STP?
Zn HCl  ZnCl H2
79.1 g
2.25 mol
? L

6. A. Limiting Reactants Zn + 2HCl  ZnCl2 + H2 79.1 g 2.25 mol ? L 79.1
g Zn
1 mol Zn
65.39
g Zn
1 mol H2 Zn
22.4 L H2
1 mol
= 27.1 L H2

7. A. Limiting Reactants Zn + 2HCl  ZnCl2 + H2 79.1 g 2.25 mol ? L
2.25 mol HCl
1 mol H2
2 mol
HCl
22.4
L H2
1 mol H2
= 25 L H2

8. A. Limiting Reactants Zn: 27.1 L H2 HCl: 25 L H2
Limiting reactant: HCl
Excess reactant: Zn
Theoretical Yield: 25 L H2
left over zinc

9. Determined by limiting reactant
B. Percent Yield
measured in lab
calculated on paper
Determined by limiting reactant

10. B. Percent Yield
When 45.8 g of K2CO3 was reacted with excess HCl, 46.3 g of KCl was formed.
a)Calculate the theoretical yield of KCl
b)Calculte the % yield
K2CO3 + 2HCl  2KCl + H2O + CO2
? g
45.8 g
actual: 46.3 g

11. B. Percent Yield K2CO3 + 2HCl  2KCl + H2O + CO2 45.8 g ? g
actual: 46.3 g
Theoretical Yield:
45.8 g
K2CO3
1 mol
K2CO3 g
2 mol
KCl
1 mol
K2CO3
74.55
g KCl
1 mol
KCl
= 49.4
g KCl

12. B. Percent Yield K2CO3 + 2HCl  2KCl + H2O + CO2 45.8 g 49.4 g
actual: 46.3 g
Theoretical Yield = 49.4 g KCl
46.3 g
49.4 g
% Yield =
 100 =
93.7%

13. Finding the Amount of Excess Reactant Left Over
calculate the amount of the excess reactant used
Set up another stoich problem.
Start with the LIMITING REACTANT and solve for the EXCESS REACTANT
subtract the amount of excess reactant used from the amount given to find the amount left over.
Can we find the amount of excess reactant in the next problem?

14. 4 Na (s) + O2(g)  2 Na2O (s)
5.00 g of sodium reacted with 5.00 g of oxygen
a. How many grams of product can form?
b. What is the limiting reactant?
c. How much excess reactant is left over? (hint: first find the amount of excess reactant used in the reaction, and subtract from the amount given)

15. 6.74 g sodium oxide Na
3.26 g of oxygen was left over