Post Lab for Limiting Reactants S’mores Lab
Name
Block
In the S’mores Lab you learned how to use mole ratios to find the “ingredient” that would
“run out” first or the LIMITING REACTANT. You also found that the limiting reactant would
determine your THEORETICAL YIELD. The ingredients you had left over were you EXCESS
REACTANTS.
You can summarize the steps for finding the limiting reactant and percent yield as follows:
A Reactant 1
+
B Reactant 2

C Product 1
+
D Product 2
Steps for determining the limiting reactant
1.
2.
3.
4.
Write down the given amounts of reactant.
Convert this quantity to moles (if not given already) using the molar mass.
Use mole ratios to find the moles of product that will be formed.
Whichever reactant produces the LEAST amount of product is the limiting reactant.
The yield of the product from the limiting reactant is the theoretical yield. All other
reactants are in excess.
5. To determine the amount of excess reactant left over:
(Amount of excess reactant you STARTED with) - (amount of excess reactant USED)
6. To find the % yield of an EXPERIMENT:
[(actual yield of experiment) / (theoretical yield )] X 100%
1
Using your previous experience, see if you can answer the following questions given this
reaction:
N2 (g) + 3 H2 (g)  2 NH3 (g)
1. If you are given the following amount of each reactant
2 mol N2
2 mol H2
a. What is the maximum amount (moles) of NH3 that you will theoretically
be able to make? (Hint… look closely at the mole ratios!)
This is your THEORETICAL YIELD.
b. What reactant did you run out of first?
(Remember, this is the LIMITING REACTANT.)
c. How many moles of the EXCESS REACTANT are left over?
2
2. If you are given the following amount of each reactant
50 g N2
20 g H2
a. What is the maximum amount (moles) of NH3 that you will theoretically
be able to make? This is your THEORETICAL YIELD.
b. What reactant did you run out of first?
(Remember, this is the LIMITING REACTANT.)
c. How many moles of the EXCESS REACTANT do you have left over?
d. If you ACTUALLY made 2.9 mol NH3, what is your % yield?
Was this a “good” experiment?
3
3. Use the following reaction to answer the questions that follow.
CO (g) + 4 H2 (g)  CH3OH (g)
You are given 67.4 g CO and 35.7 g H2.
a. How many grams of CH3OH will be produced?
b. What is the limiting reactant?
c. How much excess reactant is left over?
d. How much CH3OH is produced if the % yield of the reaction is 87.6%?
4