Chemistry Reference
In-Depth Information
●
Calculating moles
You have already seen in Chapter 3 how we can
compare the masses of all the other atoms with
the mass of carbon atoms. This is the basis of the
relative atomic mass scale
. Chemists have found by
experiment that if you take the relative atomic mass
of an element in grams, it always contains 6 × 10
23
or one mole of its atoms.
108
27
=
= 4 moles
b
number of moles of aluminium
mass of aluminium
mass of 1 mole of aluminium
=
13.5
27
=
Moles and elements
For example, the relative atomic mass (
A
r
) of iron is
56, so one mole of iron is 56 g. Therefore, 56 g of
iron contains 6 × 10
23
atoms.
The
A
r
for aluminium is 27. In 27 g of aluminium
it is found that there are 6 × 10
23
atoms. Therefore,
27 g of aluminium is one mole of aluminium atoms.
The mass of a substance present in any number of
moles can be calculated using the relationship:
mass
(in grams)
= 0.5 mole
Moles and compounds
The idea of the mole has been used so far only with
elements and atoms. However, it can also be used
with compounds (Figure 4.4).
number
×
mass of 1 mole
of moles of the element
=
Example 1
Calculate the mass of
a
2 moles and
b
0.25 mole of
iron. (
A
r
: Fe = 56)
a
mass of 2 moles of iron
= number of moles × relative atomic mass (
A
r
)
= 2 × 56
= 112 g
b
mass of 0.25 mole of iron
= number of moles × relative atomic mass (
A
r
)
= 0.25 × 56
= 14 g
If we know the mass of the element then it is
possible to calculate the number of moles of that
element using:
mass of the element
mass of 1 mole of that element
number of moles =
Figure 4.4
One mole of water (H
2
O) (left) and one mole of ethanol
(C
2
H
5
OH) (right) in separate measuring cylinders.
Example 2
Calculate the number of moles of aluminium present
in
a
108 g and
b
13.5 g of the element. (
A
r
: Al = 27)
a
number of moles of aluminium
mass of aluminium
mass of 1 mole of aluminium
What is the mass of 1 mole of water (H
2
O)
molecules? (
A
r
: H = 1; O = 16)
From the formula of water, H
2
O, you will see that
1 mole of water molecules contains 2 moles of
hydrogen (H) atoms and 1 mole of oxygen (O)
=
