Chapter 9 –
STOICHIOMETRY
The MATH of a CHEMICAL
REACTION
9.1 – Reaction Stoichiometry
-Ratio of compounds in a reaction
-YOU NEED A BALANCED CHEMICAL
REACTION!!
Examples:
Al +
O2 
Al2O3
What this means:
4 MOLES Al react with 3 MOLES O2 to
form 2 MOLES Al2O3
9.1 – Reaction Stoichiometry
Example:
C 2H 6 +
O2 
CO2 +
H 2O
2 moles C2H6 reacts with 7 moles O2 to
form 4 mole CO2 and 6 moles H2O
9.2 – Stoichiometry Calculations
-You need to be sure your units are canceling algebra!!
CAN ONLY SWITCH FROM ONE
SUBSTANCE TO ANOTHER BY
MOLES!!
9.2 – Stoichiometry Calculations
Types of Calculations:
1. mole of one substance to mole of another
Moles A
Ratio from Balanced
Equation
Moles B
Example:
4 NH3 + 3 O2  2 N2 + 6 H2O
9.4 mole NH3 = ? mole N2
1. Start with given
2. Multiply by a conversion factor
3. Conversion factor – mole ratio between substances
4. Plug into calculator
5. Remember units!!!
Work:
More Practice:
9.2 – Stoichiometry Calculations
Types of Calculations:
2. mole of one substance to gram of another
Moles A
Ratio from
Balanced
Equation
Moles B
Molar Mass from
Periodic Table
Grams B
Example:
4 NH3 + 3 O2  2 N2 + 6 H2O
4.35 mole NH3 = ? g H2O
Work:
More Practice:
9.2 – Stoichiometry Calculations
Types of Calculations:
3. gram of one substance to mole of another
Ratio from
Balanced
Equation
Molar Mass from
Periodic Table
Grams A
Moles A
Moles B
Example:
4 NH3 + 3 O2  2 N2 + 6 H2O
19.3 g NH3 = ? mol O2
Work:
More Practice:
9.2 – Stoichiometry Calculations
Types of Calculations:
4. gram of one substance to gram of another
Molar Mass from
Periodic Table
Grams
A
Moles
A
Ratio from
Balanced
Equation
Moles
B
Molar Mass from
Periodic Table
Grams
B
Example:
4 NH3 + 3 O2  2 N2 + 6 H2O
18.6 g O2 = ? g N2
Work:
More Practice:
More Practice:
Limiting Reactants
-Need a balanced equation!
-You will be given information about more than one
starting material
-Limiting Reactant = The reactant which will run
out first
-Excess Reactant = The reactant which you have
“more than enough of”
Limiting Reactants
Examples:
-Making a Bicycle:
1
1
Stockroom
Inventory:
+2
frame
10
+
2
1
+ 1
tires
18
+ 1
handlebars
13
1
bike
Limiting Reactants
Examples:
-Making a Bicycle: How many bikes can we make???
1
1
+2
frame
10
+
2
1
+ 1
tires
18
+ 1
handlebars
13
How many bikes COULD we make from each component?
10
9
13
1
bike
Limiting Reactants
How many maximum could be made??
9
*We picked the smallest value of the “could make” answers!!
Which was the limiting reactant??
What are the excess reactants??
tires
frames and handlebars
Limiting Reactants
Now a Chemistry Example:
3Fe + 4H2O  Fe3O4 + 4H2
Starting materials:
24 g Fe and 20 g H2O
1.) What is the LR?
2.) How many grams Fe3O4 will be produced?
Work:
Percent Yield
Compares the amount actually received in lab to the
amount that was expected from math problem.
Theoretical Yield = The amount expected. The
number you calculate with pen and paper (stoich
problem.)
Actual Yield = The amount actually obtained (will be given!!!)
% Yield =
actual yield
x 100
theoretical yield
Percent Yield
Example:
Easy
Harder:
3Fe + 4H2O  Fe3O4 + 4H2
Starting materials:
14.2 g Fe and 27.8 g H2O
After the experiment is complete:
17.3 g Fe3O4 are actually obtained
What is % yield?
Percent Yield
Work:
3Fe + 4H2O  Fe3O4 + 4H2
14.2 g Fe
27.8 g H2O
% Yield