Question

Explain the Law of Multiple Proportions?

Answer 1

The law of multiple proportions is one of the basic laws in chemistry, and is a major tool of chemical measurement (stoichiometry).

This law states that when elements combine they do so in a ratio of small whole numbers. For example, carbon and oxygen react to form CO or CO2, but not CO1.3 for instance. Furthermore, it states that if two elements form more than one compound between them then the ratios of the masses of the second element combined with a fixed mass of the first element will also be in ratios of small whole numbers.

The English chemist John Dalton first expressed this observation in 1803 and it is sometimes called Dalton's Law (although this term usually refers to his law of partial pressures).

Answer 2

CHECK THIS EXAMPLE

Two different compounds are formed by the elements carbon and oxygen. The first compound contains 42.9% by mass carbon and 57.1% by mass oxygen. The second compound contains 27.3% by mass carbon and 72.7% by mass oxygen. Show that the data are consistent with the Law of Multiple Proportions.

SOLUTION

The Law of Multiple Proportions is the third postulate of Dalton's atomic theory. It states that the masses of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.

Therefore, the masses of oxygen in the two compounds that combine with a fixed mass of carbon should be in a whole-number ratio. In 100 g of the first compound (100 is chosen to make calculations easier) there are 57.1 g O and 42.9 g C. The mass of O per gram C is:

57.1 g O / 42.9 g C = 1.33 g O per g C

In the 100 g of the second compound, there are 72.7 g O and 27.3 g C. The mass of oxygen per gram of carbon is:

72.7 g O / 27.3 g C = 2.66 g O per g C

Dividing the mass O per g C of the second (larger value) compound:

2.66 / 1.33 = 2

Which mean that the masses of oxygen that combine with carbon are in a 2:1 ratio. The whole-number ratio is consistent with the Law of Multiple Proportions.