Chemistry Reference
In-Depth Information
Example 3
Calculate the mass of
a
3 moles and
b
0.2 moles of
carbon dioxide gas, CO
2
. (
A
r
: C = 12; O = 16)
a
One mole of CO
2
contains 1 mole of carbon
atoms and 2 moles of oxygen atoms. Therefore:
mass of 1 mole of CO
2
= (1 × 12) + (2 × 16)
= 44 g
mass of 3 moles of CO
2
= number of moles × mass of 1 mole of CO
2
= 3 × 44
= 132 g
b
mass of 0.2 mole of CO
2
= number of moles × mass of 1 mole of CO
2
= 0.2 × 44
= 8.8 g
If we know the mass of the compound then we can
calculate the number of moles of the compound
using the relationship:
atoms. The mass of 1 mole of water molecules is
therefore:
(2 × 1) + (1 × 16) = 18 g
The mass of 1 mole of a compound is called its
molar mass. If you write the molar mass of a
compound without any units then it is the relative
formula mass, often called the
relative molecular
mass
(
M
r
). So the relative formula mass of water
is 18.
Now follow these examples to help you learn and
understand more about moles and compounds.
Example 1
What is
a
the mass of 1 mole and
b
the relative
formula mass (RFM) of ethanol, C
2
H
5
OH?
(
A
r
: H = 1; C = 12; O = 16)
a
One mole of C
2
H
5
OH contains 2 moles of carbon
atoms, 6 moles of hydrogen atoms and 1 mole of
oxygen atoms. Therefore:
mass of 1 mole of ethanol
= (2 × 12) + (6 × 1) + (1 × 16)
= 46 g
b
The RFM of ethanol is 46.
mass of compound
mass of 1 mole of the compound
number of moles
of compound
=
Example 4
Calculate the number of moles of magnesium oxide,
MgO, in
a
80 g and
b
10 g of the compound.
(
A
r
: O = 16; Mg = 24)
a
One mole of MgO contains 1 mole of magnesium
atoms and 1 mole of oxygen atoms. Therefore:
Example 2
What is
a
the mass of 1 mole and
b
the RFM of
nitrogen gas, N
2
? (
A
r
: N = 14)
a
Nitrogen is a diatomic gas. Each nitrogen
molecule contains two atoms of nitrogen.
Therefore:
mass of 1 mole of N
2
= 2 × 14
= 28 g
b
The RFM of N
2
is 28.
The mass of a compound found in any number of
moles can be calculated using the relationship:
mass of
compound
mass of 1 mole of MgO
= (1 × 24) + (1 × 16)
= 40 g
number of moles of MgO in 80 g
mass of MgO
mass of 1 mole of MgO
=
80
40
=
=
×
number of moles
of the compound
mass of 1 mole
of the compound
= 2 moles
b
number of moles of MgO in 10 g
mass of MgO
mass of 1 mole of MgO
=
10
40
=
= 0.25 mole
