Law of Constant Composition

Proust established this law, in 1799. It states that all pure samples of the same compound contain the same elements combined in the same proportion by mass. For example, a pure sample of water irrespective of its source contains 88.89 mass % of oxygen and 11.11 mass % of hydrogen.

Experimental verification:
Prepare pure samples of cupric oxide by two different methods:
1. by heating copper carbonate
2. by the decomposition of cupric nitrate
The cupric oxide prepared by both the methods always contains the same elements copper and oxygen combined together in the same fixed proportion of 4:1 by weight.

Calculation:
Weight of cupric oxide = W1 gm
Weight of copper = W2 gm
Weight of oxygen = W1-W2 gm
Ratio of copper: oxygen = W2: (W1-W2)
The same experiment is repeated with a known weight of W3 gm of copper oxide prepared by heating copper carbonate.

CuCO3 CuO + CO2
The cupric oxide formed is reduced to metallic copper by passing a current of pure and dry hydrogen inside the tube as before. The weight of metallic copper was found to be W4gm. The ratio of the weight of copper to the weight of oxygen in both the samples are calculated as follows:

Weight of cupric oxide = W3gm.
Weight of copper = W4 gm
Weight of oxygen = W3 -W4 gm
Ratio of copper to oxygen = W4: (W3 -W4)
The two ratios are found to be the same and is equal to 4:1.Thus the law of definite proportions is verified experimentally.