Law of multiple proportions:
This law was put forth by Dalton in 1808. According to this law, if two elements combine to form more than one compound, then the different masses of one element which combine with a fixed mass of the other element, bear a simple ratio to one another.
Example:
1)Hydrogen and oxygen combine to form two compounds H2O (water) and H2O2 (Hydrogen peroxide).
In water (H2O): 2g of hydrogen combines with 16g of oxygen. In H2O2: 2g of hydrogen combines with 32g of oxygen.
Therefore the masses of oxygen which combines with same mass of hydrogen in these two compounds bear a simple ratio of 1:2.
2)Nitrogen and oxygen combine and form compounds like N2O, NO, NO2 et3)
In N2O: 28 g of nitrogen combines with 16 g of oxygen
Therefore, 14 g of nitrogen combines with 8 g of oxygen
NO : 14 g of nitrogen combines with 16 g of oxygen
NO2: 14 g of nitrogen combines with 32 g of oxygen
Therefore, the masses of oxygen which combine with same mass of nitrogen in the three compounds bear a ratio of 8:16:32 (or) 1:2:3.