LAW OF PROPORTION IN CHEMISTRY

       Define the law of proportion?

Ans :-

Elements in a compound always occur in the combined state in definite proportion by weight .

                                               Or

A chemical compound has the same composition , by whatever method it is prepared.

Example Law of Multiple Proportions Problem:

Two different compounds are formed by the elements carbon and oxygen.

The first compound contains 42.9% and carbon 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

This is the  Law of Multiple Proportions is the third postulate of Dalton's atomic theory.

It states that the mass of one element which combine with a fixed mass of the second element are in a ratio of whole numbers.

So  , 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 used  to do 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

Its  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.