Law of definite composition

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The law of definite proportions (or Proust's law) states that a chemical compound always has exactly the same proportion of elements by mass.

Amounting to the same thing is the law of constant composition. This states that all samples of a given chemical compound have the same elemental composition by mass. For example, oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. The law of definite proportions is the basis of stoichiometry, together with the law of multiple proportions.