State the law of definite proportions. Suggest one problem to understand the law by working out that problem.

Explanation

Law of definite proportions : "A given chemical substance always contains the same elements combined in a fixed proportion by weight."
Explanation : `SO_(2)` can be obtained by the following two methods.
i) When mercuric sulphide is heated in air, it gives mercury and sulphur dioxide.
`HgS+O_(2)toHg+SO_(2)`
ii) When lead sulphide is heated strongly in air, it gives lead oxide and sulphur dioxide.
`2PbS+3O_(2)to2PbO+2SO_(2)`
Samples of `SO_(2)` obtained by the above two methods were analysed. In each of them, 100 g of `SO_(2)` was found to contain 50 g of sulphur and 50 g of oxygen.
The above observations prove that the weight composition of sulphur dioxide is always constant.