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Stoichiometry
Section 12.1
What is Stoichiometry?
Study of quantitative relationships between amounts of
reactants used and products formed.
Based on the Law of Conservation of Mass (Mass of the
Reactants = Mass of the Products)
Ex. 4 Fe (s) + 3 O 2 (g)  2 Fe2O3 (s)
Mole Ratios
Ratio between the numbers of moles of any two substances
in a balanced chemical equation.
Ex. 2 Al (s) + 3 Br2 (l)  2 AlBr3 (s)
Mole Ratios:
2 Al : 3 Br2
2 Al : 2 AlBr3
3 Br2: 2 AlBr3
Mole Ratio Practice
What mole ratios can be written for the decomposition of
potassium chlorate?
Answer:
2 KClO3 (s)  2 KCl (s) + 3 O2 (g)
2 KClO3 : 2 KCl
2 KClO3 : 3 O2
2 KCl
: 3 O2
Mole – Mole Conversions
• The problem gives you moles and asks for moles as an
answer
1. Balance the equation
2. Label the known substance (given in the problem) and
the unknown substance (substance being asked for in
the problem)
3. Determine the mole ratio that relates the known and
unknown substances
4. Multiply the known number of moles by the mole ratio
(H) Mole – Mole Conversion
Practice
2 K (s) + 2 H2O (l)  2 KOH (aq) + H2 (g)
How many moles of hydrogen are produced when .0400 mole of
potassium is used?
1. Write the balanced equation
2. Given
= .0400 mole K
Unknown = moles of H2
3. Write the mole ratio:
1 mole H2
2 mole K
4. Convert using: moles of known x moles of unknown
moles of known
Answer: .0400 x 1 mole H2 = .0200 mol H2
2 moles K
Stoichiometry Practice I
(Mole-Mole Conversions)
H2
+
O2

H2O
8. How many moles of water can be produced with 6 moles
of hydrogen?
9. How many moles of oxygen would be required to fully
react with 8 moles of hydrogen?
10. How many moles of water can be produced with 4
moles of oxygen?
11. How many moles of hydrogen would be required to
produced 10 moles of water?
12. How many moles of oxygen would be needed to
produce 20 moles of water?
Mole – Mass Relationships
Coefficients in a balanced equation can be interpreted in
terms of representative particles and also by numbers of
moles of particles.
We can also use what we know about the conversion factor
that relates mass and number of moles to find the mass of
either the reactants or products.
How to solve mole to mass
problems

1.
2.
3.
4.
5.
Problem gives you moles and asks for mass as an
answer
Balance equation
Label the known substance and the unknown
substance
Determine the mole ratio that relates the known
and unknown substances
Multiply the known number of moles by the mole
ratio
Multiply the number of moles by the molar mass of
the unknown substance
Mole – Mass Relationships
Ex. 4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)
4 mol Fe x 56 g Fe = 224 g Fe
1 mol Fe
3 mol O2 x 32 g O2 = 96 g O2
1 mol O2
Mass of Reactants
= 320 g
2 mol Fe2O3 x 160 g Fe2O3 = 320 g
1 mol Fe2O3
Mass of Products
= 320 g
Mass to Mass Relationships in
Equations

Problem gives you mass and asks for mass
as an answer
1.
Balance equation
Label the known substance and the
unknown substance
Convert from the mass to the moles
Use a mole ratio
Use the molar mass to convert from moles
to mass
2.
3.
4.
5.
(CP) Mole – Mole Relationships
in Equations Example
Example:
2 H2 + O2  2 H2O
How many moles of water can be produced with 6 moles of
hydrogen?
1. Equation is balanced.
2. There are 2 moles of hydrogen, 1 mole oxygen, and 2
moles water.
3. 6 H2 = x H2O
6 = x and 2x = 12
2 H2
2 H2O
2 2
x=6
4. X = 6 moles water
(CP) Mole – Mole Relationships
in Equations
1. Balance the equation.
2. Coefficients = number of moles of each substance.
3. Use “x” for the “how many” compound.
4. Use ratio of moles given in problem to actual moles in
equation.
5. Set up ratio.
6. Solve.
(CP) Mole to Mass Relationships
in Equations
Balance the equation.
2.
Coefficients = # of moles of each substance.
3.
Use “x” for the “how many” compound.
4.
Use ratio of moles given in problem to actual moles
in equation.
5.
Set up ratio and solve for “x”.
6.
Multiply your answer by the molar mass of the
element or compound you are trying to find.
(Example: Stoichiometry Practice I, # 4 at bottom)
1.