Two samples X and Y of a pure substance obtained by two different methods contain elements P and Q. Substance X has a mass of 2.25 g and contains 0.90 g of O. Substance Y is made up of 60% P and

40% O by weight. This is an illustration of

Law of constant proportions

Mass of  $X=2.25g$

Mass of $P=2.25-0.90=1.35g$

Ratio of masses of P and Q in sample

$X=1.35:0.90i.e.1.5:1$

Ratio of masses of P and Q in sample 

$Y=60:40i.e.1.5:1$

As the ratio of the masses of P and Q in two samples X and Y is constant i.e. 1.5 : 1 so, this is on illustration of law constant proportions.