Law of definite proportions

A chemical compound contains the same elements in fixed proportions by mass, regardless of its source or method of preparation.

Atomic theory: The theory that matter consists of atoms, which combine in definite ways to form substances. Its account of compounds as combinations of atoms explains fixed elemental ratios.
John Dalton: An English chemist who developed an early modern atomic theory in the early nineteenth century. Dalton used fixed atomic combinations to account for constant compound compositions.
Joseph Proust: A French chemist who formulated the law of definite proportions from studies of chemical composition. Proust argued from analyses that compounds retain constant composition across preparations.
Empirical formula: The simplest whole-number ratio of atoms of each element in a compound. Mass composition can be converted into the simplest atomic ratio for an unknown compound.
Mixture: A physical combination of substances whose components retain their chemical identities and can occur in variable proportions. Mixtures can change composition freely, unlike a pure compound governed by the law.
Chemical compound: A pure substance made of two or more elements chemically bonded in fixed proportions. The law applies to compounds, not mixtures whose composition can vary.
Atomic mass: A measure of an atom’s mass, expressed relative to a standard based on carbon-12. Elemental atomic masses determine the mass ratios implied by a compound’s formula.
Antoine Lavoisier: A French chemist whose quantitative methods helped establish modern chemistry in the late eighteenth century. His precise weighing practices made comparisons of compound composition more reliable.
Elemental analysis: The measurement of the amounts of elements present in a chemical sample. Repeated elemental measurements test whether a sample has a compound’s expected composition.
Law of reciprocal proportions: A law relating the masses of two elements that combine separately with a fixed mass of a third element. It is another classical composition law, but compares relationships across different compounds.

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