Law of definite proportions
One of the fundamental observations of modern chemistry made by Joseph Louis Proust, the law of definite proportions, states that, in a sample of a pure compound, the elements combine in definite proportions to each other by mass. This is considered one of the fundamental chemical laws. It is also known as the "law of constant composition", alluding to the fact in every sample of a pure chemical compound the elements combine in the same definite proportion.
For example, water is a pure compound, made up of hydrogen and oxygen. In any given sample of pure water, there will always be two hydrogen atoms for every single oxygen atom, and the mass ratio will always be 88.81% Oxygen to 11.20% Hydrogen.
(This is not to say, however, that all compounds of hydrogen and oxygen combine in this proportion. Hydrogen peroxide, for example, is a compound of two hydrogen atoms for every two oxygen atoms. For more about this, see Law of multiple proportions.)
There is also a whole class of substances, called non-stoichiometric compounds (also called Berthollides), which do not follow this law. For these componds, the ratio between the elements can vary continuously within certain limits. Ferrous oxide is an example of a berthollide. The ideal formula is FeO, but due to crystallographic vacancies it is reduced to about Fe0.95O.