Stoichiometry
Please review the “Moles Conversion Examples” document found on the message board for Module 2 and the “Balancing
Equations” document found on the Module 4 message board.
Steps for Stoichiometry problems…
1) Balance the equation. (You need to know the ratios.)
2) Set up as:
What you are given x
What you want
= What you want
What you are given
This may take several conversions to get to your final answer.
3) Multiply numerators (top numbers) then divide by the denominators (bottom numbers).
4) Make sure you have labels on ALL values. This includes the units (grams or moles) AND the substance formula.
Given: MOLES -> Want: MOLES
These types of problems start with moles of one substance and want you to tell the moles of another substance from the
reaction.
EXAMPLE: How many moles of oxygen gas are needed to produce 2.50 moles of water?
1) Balance the equation: H2 + O2 = H20
2 H2 + O2
H-2x2=4√
O-2√
2) Set up as:
Given
2.5 mol H2O
(What you are given)
2 H2O
H-2x2=4√
O-2√
x
x
The ratio of O2:H2O is 1:2
Mole ratio from balanced
equation.
1 mol O2
(What you want)
2 mol H2O
(What you are given)
=
mol O2
=
1.25 mol O2
(What you want)
3) (2.5 x 1)/2 = 1.25
4) Notice all values in step #2 have units and formulas for the substance!! You need to give the balanced equation and
set up with final answer to properly show your work for these types of problems.
Given: MOLES -> Want: MASS
These types of problems start with moles of one substance and want you to tell the grams (mass) of another substance
from the reaction.
EXAMPLE: How many grams of sodium metal are needed to react completely with 3.00 moles of water?
1) Balance the equation: Na + H2O  NaOH + H2
2 Na + 2 H2O
Na – 1 x 2 = 2 √
H-2x2=4√
O–1x2√
2)
2 NaOH + H2
Na – 1 x 2 = 2√
H – (1 x 2) + 2 = 4 √
O–1x2=2√
Set up as:
Given
x
3 mol H2O
(What you are given)
x
Mole ratio from
balanced equation.
2 mol Na
(What you want)
2 mol H2O
(What you are given)
The ratio of Na:H2O is 2:2
x
x
Molar mass of Na
22.99 grams Na
(What you want)
1 mol Na
(What you are given)
=
grams Na
=
68.97 grams Na
(What you want)
3) (3 x 2 x 22.99)/(2 x 1) = 68.97
4) Notice all values in step #2 have units and formulas for the substance!! You need to give the balanced equation and
set up with final answer to properly show your work for these types of problems.
Given: MASS -> Want: MOLES
These types of problems start with grams (mass) of one substance and want you to tell the moles of another substance
from the reaction.
EXAMPLE: How many moles of magnesium metal would be needed to react completely with 34.7 grams of sodium
fluoride?
1)
Balance the equation: Mg + 2 NaF → MgF2 + 2 Na
Mg + 2 NaF
Na - 1 x 2 = 2 √
Mg - 2 √
F–1x2√
2)
Set up as:
Given
34.7 g NaF
(What you are given)
x
x
The ratio of Mg:NaF is 1:2
MgF2 + 2 Na
Na - 1 x 2 = 2√
Mg - 1 √
F-2√
Molar mass of NaF
1 mol NaF
(What you want)
41.98 grams NaF
(What you are given)
x
x
Mole ratio from
balanced equation.
1 mol Mg
(What you want)
2 mol NaF
(What you are given)
=
mol Mg
=
0.413 mol Mg
(What you want)
3) (34.7 x 1 x 1)/41.98 x 2) = 0.413
4) Notice all values in step #2 have units and formulas for the substance!! You need to give the balanced equation and
set up with final answer to properly show your work for these types of problems.
Given: MASS -> Want: MASS
These types of problems start with grams (mass) of one substance and want you to tell the moles of another substance
from the reaction.
EXAMPLE: If 35.5 grams of calcium oxide react with excess water, how many grams of calcium hydroxide can be formed?
1) Balance the equation: CaO + H2O → Ca(OH)2 IT IS BALANCED!! 
CaO + H2O
Ca - 1
O-2
H-2
Ca(OH)2
Ca - 1
O-2
H-2
The ratio of CaO: Ca(OH)2 is 1:1
2) Set up as:
Given
35.5 g CaO
(What you are given)
x
x
Molar mass of CaO
1 mol CaO
(What you want)
56.077 grams CaO
(What you are given)
x
Mole ratio from
balanced equation.
1 mol Ca(OH)2
x
(What you want)
1 mol CaO
(What you are given)
x
x
Molar mass of
Ca(OH)2
74.0918 g Ca(OH)2
(What you want)
1 mol Ca(OH)2
(What you are given)
=
mol Mg
=
46.9 g Ca(OH)2
(What you want)
3) (35.5 x 1 x 1 x 74.0918)/(56.077 x 1 x 1) = 46.
4) Notice all values in step #2 have units and formulas for the substance!! You need to give the balanced equation
and set up with final answer to properly show your work for these types of problems.