Chapter 12:
Stoichiometry
Limiting Reagent and
Percent Yield
Learning Target
You will determine the limiting reactant in
a problem, the theoretical yield, and the
amount of excess reactant remaining
when the reaction is completed.
You will calculate the percent yield of a
reaction.
Limiting and Excess Reagents
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Limiting Reagent Problem Example
Copper reacts with sulfur
to form copper (I) sulfide
according to the following
balanced equation:
2 Cu (s) + S (s)  Cu2S (s)
What is the maximum
number of grams of Cu2S
that can be formed when
80.0 g Cu reacts with 25.0
g S?
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
How to solve
Pick a given amount to start with
and perform a stoichiometric
calculation to figure out
theoretical yield of other
reactant.
2. Compare theoretical yield to
given amount of reactant
If theoretical yield > given yield,
then LR
If theoretical yield < given yield,
then ER
(make sure units are same)
3. Use LR as given value and solve
as we’ve been
1.
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Shortcut (ONLY for LR)
1. Convert the amount of each reactant to
moles (if not already)
2. Divide each mole amount by its
respective coefficient
3. Smallest number is the limiting reactant
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Example #1
Copper reacts with sulfur to
form copper (I) sulfide
according to the following
balanced equation:
2 Cu (s) + S (s)  Cu2S (s)
What is the maximum number
of grams of Cu2S that can be
formed when 80.0 g Cu
reacts with 25.0 g S?
 100. g Cu2S
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Example #1 (work)
2 Cu (s) + S (s)  Cu2S (s)
What is the maximum number of grams of Cu2S that can be
formed when 80.0 g Cu reacts with 25.0 g S?
 100. g Cu2S
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Example #2
The heat from an acetylene
torch is produced by burning
acetylene (C2H2) in oxygen.
2 C2H2 (g) + 5 O2 (g) 
4 CO2 (g) + 2 H2O (g)
How many grams of water
can be produced by the
reaction of 2.40 mol C2H2
with 7.40 mol O2?
43.2 g H2O
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Example #2 (work)
2 C2H2 (g) + 5 O2 (g)  4 CO2 (g) + 2 H2O (g)
How many grams of water can be produced by the
reaction of 2.40 mol C2H2 with 7.40 mol O2?
43.2 g H2O
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Try on your own
Aluminum reacts with chlorine gas to form
aluminum chloride according to the balanced
equation
2 Al (s) + 3 Cl2 (g)  2 AlCl3 (s)
In a certain experiment, 10.0 g of aluminum is
reacted with 35.0 g of chlorine gas. What mass
of aluminum chloride will be produced, assuming
a complete reaction?
• = 43.9 g AlCl3
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Try on your own
1st-Find the LR
2 Al (s) + 3 Cl2 (g)  2 AlCl3 (s)
10.0 g 35.0 g
?g
10.0 g Al x 1 mol Al x 3 mol Cl2 x 70.90 g Cl2
1
26.98gAl
2molAl 1molCl2
= 39.4 g Cl2 NEEDED…> 35.0 g Cl2 GIVEN,
LR
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Try on your own (cont.)
2nd-Find the mass of AlCl3
2 Al (s) + 3 Cl2 (g)  2 AlCl3 (s)
10.0 g 35.0 g
?g
35.0 gCl2 x 1molCl2 x 2molAlCl3 x133.33 gAlCl3
1
70.90 g Cl2 3 mol Cl2 1molAlCl3
= 43.9 g AlCl3
You will determine the limiting reactant in a problem, the theoretical yield, and the
amount of excess reactant remaining when the reaction is completed.
Percent Yield
What is the percent yield if 13.1 g CaO
is actually produced when 24.8 g
CaCO3 is heated?
CaCO3 (s)

CaO (s) +
CO2 (g)
You will calculate the percent yield of a reaction.
Example
What is the percent yield if 13.1 g CaO
is actually produced when 24.8 g
CaCO3 is heated?
CaCO3 (s)

CaO (s) +
CO2 (g)
 94.2%
You will calculate the percent yield of a reaction.
Example (work)
What is the percent yield if 13.1 g CaO is actually
produced when 24.8 g CaCO3 is heated?
CaCO3 (s)

CaO (s) +
CO2 (g)
 94.2%
You will calculate the percent yield of a reaction.
Try on your own
If 5.00 g of silicon dioxide is heated with an
excess of carbon, 27.9 g of silicon carbide is
produced.
SiO2 (s) + 3 C (s)  SiC (s) + 2 CO (g)
What is the percent yield of this reaction?
 83.5%
You will calculate the percent yield of a reaction.
Try on your own (work)
If 5.00 g of silicon dioxide is heated with an excess of
carbon, 27.9 g of silicon carbide is produced.
SiO2 (s) + 3 C (s)  SiC (s) + 2 CO (g)
What is the percent yield of this reaction?
 83.5%
You will calculate the percent yield of a reaction.
Quiz
Consider the following chemical reaction:
3 H2 + N2  2 NH3.
If you are given 6 molecules of H2 and 4 molecules of N2, what
is the limiting reactant?
2. Consider the following chemical reaction:
2 KCl + 3 O2  2 KClO3.
If you are given 100.0 moles of KCl and 100.0 moles of O2,
what is the limiting reactant?
3. Explain what a limiting reactant is to a student who has
been absent from this class for a few days.
4. If 15.0 g of nitrogen reacts with 15.0 g of hydrogen, 10.5 g
of ammonia is produced. What is the percent yield of this
reaction? (use the equation in #1)
1.
You will determine the limiting reactant in a problem, the theoretical yield, and the amount of excess reactant
remaining when the reaction is completed.
You will calculate the percent yield of a reaction.