The law of multiple proportion states that when two elements combine to form two or, more than two compounds, the weights of one of the elements which combine with a fixed weight of the other, bear a simple whole number ratio.
The law may be illustrated by the following example : Nitrogen and oxygen combine to form five oxides namely nitrous oxide (N2O), nitric oxide (NO), nitrogen trioxide (N2O3), nitrogen tetroxide (N2O4) and nitrogen pentoxide (N2O5).
| Oxide |
Number of
parts by
weight of
nitrogen |
Number of
parts by
weight of
oxygen |
Fixed
weight of
nitrogen
(14 parts) |
Number of parts
by weight of
oxygen combining
with 14 parts
by weight of
nitrogen |
| NO2 |
28 |
16 |
14 |
8 |
| NO |
14 |
16 |
14 |
16 |
| N2O3 |
28 |
48 |
14 |
24 |
| N2O4 |
28 |
64 |
14 |
32 |
| N2O5 |
28 |
80 |
14 |
40 |
The ratio between the different weights of oxygen in different compounds which combine with the same weight of nitrogen (14 parts) is
8 : 16 : 24 : 32 : 40
or, 1 : 2 : 3 : 4 : 5
This is simple whole number and hence, supports the law.
