STOICHIOMETRY
436
 5.6 – To determine, using
stoichiometric calculations, the
quantity of a substance involved in
a chemical reaction.
 5.8 – To solve numerical problems
related to stoichiometry.
A mole is the amount of substance
that contains 6.02 x 1023 particles of
that substance
n (moles) =
mass of substance (g)
molar mass of substance (g/mol)

Stoichiometry:

uses ratios of a chemical reaction to
determine the amounts of elements/
compounds (reactants & products)
involved in a chemical change
Magnesium metal reacts with oxygen
to produce magnesium oxide.

2Mg
+
O2

2MgO
2
:
1
:
2
Mole Ratio –
describes the ratio in which the substances
combine
 Ex.
for every 2 Mg, 1 O2 needed

used to predict what will happen
 Ex.
for every 2 Mg & 1 O2, 2 MgO will be
produced

4 moles of magnesium metal reacts
with oxygen to produce magnesium
oxide. How many moles of MgO are
produced?
Mole Ratio
2Mg
+
O2

2MgO
2
:
1
:
2
from mole ratio
Set up ratio
2 moles Mg
4 moles Mg
from question
=
2 moles MgO
? moles MgO
Solve by cross multiplying
2 moles Mg
4 moles Mg
=
2 moles MgO
? moles MgO
4x2÷2
4 moles MgO
are produced
Iron metal reacts with oxygen to
produce 3 moles of iron (III) oxide.
How many moles of oxygen were
needed in the reaction?
Mole Ratio
4Fe
+
3O2 
2Fe2O3
4
:
3
2
from mole ratio
Set up ratio
3 moles O2
? moles O2
from question
:
=
2 moles Fe2O3
3 moles Fe2O3
Solve by cross multiplying
3 moles O2
? moles O2
=
2 moles Fe2O3
3 moles Fe2O3
3x3÷2
4.5 moles O2
are needed
How many moles of nitrogen gas &
hydrogen gas are required to make
7.5 mol of ammonia?
N2
+
3H2 
2NH3
1
:
3
2
:
Solve for moles of N2:
1 mole N2
? moles N2
=
11.25 moles H2
2 moles NH3
7.5 moles NH3
3.75 moles N2
are required
are required
Solve for moles of H2:
3 mole H2
? moles H2
=
2 moles NH3
7.5 moles NH3
We can now add mass into problems
involving stoichiometry.
Recall,
n (moles) =
mass of substance (g)
molar mass of substance (g/mol)
How many moles are in 12.5g of Ca?
n (moles) =
12.5g
40g/mol
0.3125 mol of Ca
How many moles are in 2.4g of NO2?
n (moles) =
2.4g
46g/mol
0.052 mol of NO2
How many moles of Li are needed to
react with fluorine gas to make 435g
of lithium fluoride?
2Li +
F2

2LiF
2
1
:
2
:
Solve for moles of LiF:
n (moles) = 435g
26g/mol
16.73 mol of LiF
Solve for moles of Li:
2 moles Li
? moles Li
= 2 moles LiF
16.73 mol LiF
16.73 moles Li
are needed
How many grams of NaClO3 must be
decomposed to produce sodium
chloride & 1.65 moles of oxygen?
2NaClO3 
2
:
2NaCl
2
+
:
3O2
3
Solve for moles of NaClO3:
2 mol NaClO3
? mol NaClO3
=
3 mol O2
1.65 mol O2
1.1 mol of NaClO3
Solve for mass of NaClO3:
1.1 mol =
?g
106 g/mol
116.6g of
NaClO3
3g of magnesium react with how many
grams of hydrochloric acid to produce
hydrogen gas & magnesium chloride?
Mg +
1
:
2HCl
2
Solve for moles of Mg:
n =
3g
24 g/mol
0.125 mol of Mg
Solve for moles of HCl:
1 mol Mg
= 2 mol HCl
0.125 mol Mg
? mol HCl

:
MgCl2
1
+
:
H2
1
Solve for mass of HCl:
0.25mol = ? g
36 g/mol
9g of HCl
0.25 mol of HCl

Always find moles first!!!
n (moles) =
mass of substance (g)
molar mass of substance (g/mol)


Use mole ratio to find the moles of each
reactant & product
Remember, the equation must be
balanced to give you the mole ratio

Worksheet #