Law of Constant Proportion

Another French chemist, Joseph Proust, stated this law as 'A chemical compound always contains same elements combined together in the same proportion by mass'.

For example, pure water obtained from different sources such as river, well, spring, sea, etc., always contains hydrogen and oxygen together in the ratio of 1: 8 by mass. Similarly, carbon dioxide CO2 can be obtained by different methods such as,
  • Burning of carbon
  • Heating of lime stone
  • By action of dilute HCl on marble pieces

But the different samples of CO2 always contain carbon and oxygen in the ratio of 3: 8.

Example 2 :

When 1.375g of cupric oxide was reduced on heating in a current of hydrogen, the weight of copper that remained was 1.098g. In another experiment 1.179g of copper was dissolved in nitric acid and resulting copper nitrate converted into cupric oxide by ignition. The weight of cupric oxide formed was 1.476g. Show that these results illustrate the law of constant proportion.


First experiment

Copper oxide = 1.375g

Copper left = 1.098g

Oxygen present = 1.375 - 1.098 = 0.277g

Second Experiment

Copper taken = 1.179g

Copper oxide formed = 1.476g

Oxygen present = 1.476 - 1.179 = 0.297g

Percentage of oxygen is approximately the same in both the above cases. So the law of constant composition is illustrated.

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