According to the law of definite proportions, the samples of a compound will always contain the same proportion of elements by mass. The law is the basis for the study of stoichiometry in Chemistry.
The law of definite proportions, also known as the law of constant proportions, states ‘that the individual elements that constitute a chemical compound are always present in a fixed ratio’ (in terms of their mass). This ratio does not depend on the source of the chemical compound or the method through which it was prepared.
The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass. This indicates that any pure sample of a compound, no matter the source, will always comprise of the same elements that are present in the same ratio by mass.
For example, in a nitrogen dioxide (NO2) molecule, the ratio of the number of nitrogen and oxygen atoms is 1:2 always. The ratio between the nitrogen and oxygen molecule remain the same always.