Question

State and explain the law of constant proportions by giving an example.

Answer 1

Law of constant proportions: According to the law of chemical proportions, the elements combine in a definite proportion by mass to form a chemical substance irrespective of their source or methods of formation.

• This law was given by Proust in 1797.

Explanation of the law of constant proportions:

• Consider the example of Water $\left({\mathrm{H}}_2\mathrm{O}\right)$.
• In water, the weight of Hydrogen $\left({\mathrm{H}}_2\right)$ is $2\mathrm{g}$ whereas the weight of Oxygen $\left(\mathrm{O}\right)$ is $16\mathrm{g}$.
• So the ratio in which hydrogen and oxygen have combined by mass is $2:16\mathrm{or}1:8$ by mass.
• This ratio is fixed for water. In other words, water will be water because in it hydrogen and oxygen combine in $1:8$ ratio by mass irrespective of whether that water is taken from the pond, lake, sea, etc., and whatever method is used to combine hydrogen and oxygen to produce water. This ratio will always be constant.