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From Citizendium, the Citizens' Compendium

One of the fundamental observations of modern chemistry made by Joseph Louis Proust, the law of definite proportions states that, in a pure compound, the elements combine in definite proportions to each other by mass. This is considered one of the fundamental chemical laws.

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, please see Law of multiple proportions.)

There is also a whole class of substances, called non-stoichiometric compounds (also called Berthollides), which don't follow this law. For these componds, the ratio between the elements can vary continuously between 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 Fe_0.95O.