1. Limiting Reagents and Percent Yield
Yes…it’s still stoichiometry…

2. Think about making grilled cheese…
To make a decent grilled cheese sandwich, you need two pieces of bread, 2 pieces of cheese, and 1 pat of butter. Write an equation that describes making a grilled cheese sandwich.
2 Bread + 2 Cheese + 1 Butter  1 Grilled Cheese Sandwhich

3. Grilled Cheese Example Continued
If you had 6 pieces of bread, 4 pieces of cheese, and 3 pats of butter, which of the ingredients would limit how many sandwiches could be made?
The cheese. 6 Bread could make 3 sandwiches, 3 butters could make 3 sandwiches, but 4 cheese can only make 2 sandwiches.
Which of the ingredients would be leftover?
Bread and Butter

4. Limiting and Excess Reagent
Limiting Reagent: reactant that determines the amount of product that can be formed
The reaction only occurs until the limiting reagent is used up (in our example, that was the cheese)
Excess Reagent: the reactant that is not completely used up
Some of the reactant is leftover after the reaction occurs (in our example, this was the bread and butter)

5. How to solve limiting and excess reagent problems
You will be given 2 starting points, so you will solve 2 stoichiometry problems.
Example:
Copper reacts with sulfur to form copper(I) sulfide. Identify the limiting and excess reagents when 80.0 g Cu reacts with 25.0 g S.
Cu + S  Cu2S

6. Practice Problem For each problem, indicate the limiting reagent, the excess reagent, and the amount produced.
1. The combustion of ethene is described below: C2H4 + 3O2  2CO2 + 2H2O If 2.70 mol C2H4 is reacted with 6.30 mol O2, identify the limiting reagent.

7. Another Practice Problem
2. When 6.00 g HCl reacts with 5.00 g Mg, how many grams of hydrogen gas can be produced?
2HCl + Mg  MgCl2 + H2

8. Percent Yield = Actual Yield/Theoretical Yield (100)
Theoretical Yield = what you could get according to stoichiometry
Actual Yield = what is really made in the lab (usually lower than theoretical due to error/loss of efficiency in system)
Percent Yield = Actual Yield/Theoretical Yield (100)

9. To Solve Percent Yield Problems
Calculate the theoretical yield
Will require a stoichiometry problem
May be a limiting reagent problem (aka: 2 or more stoichiometry problems)
Find the actual yield (given to you in the problem, or found during lab)
Divide the actual yield by the theoretical yield
Multiply by 100 (it’s a percentage!)

10. Example Problem
If in the following reaction g H2 produces g of water, what is the percent yield? Fe3O4 + H2  Fe + H2O

11. Different Type of Percent Yield Problem
Sometimes, you’ll know the percent yield and need to work backwards to figure out actual yield, for example:
If the reaction of 91.3 g C3H6 produces an 81.3% yield, how many grams of CO2 would be produced?
C3H O2  CO H2O