Law of definite composition
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.