AllChemicals — The Online Chemical Glossary | https://allchemicals.info/chemical/548-law-of-definite-proportions
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Also Known As
Proust's law, law of constant proportions, law of definite composition
What is Law of Definite Proportions?
Also called Proust's law, it states that a pure compound always contains the same elements in the same proportions by mass regardless of the source or method of preparation. For example, water is always 88.9% oxygen and 11.1% hydrogen by mass.
Properties & Characteristics
The Law of Definite Proportions (Proust's law, 1799) states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of the source or method of preparation. This was a key observation supporting Dalton's atomic theory: fixed ratios arise because atoms combine in whole-number ratios. Example: water always contains hydrogen and oxygen in a 1:8 mass ratio (or 2:1 molar ratio).
Uses & Applications
The law of definite proportions underpins: the concept of empirical formulas, stoichiometric calculations, quality control of chemical products (ensuring correct composition), and pharmaceutical purity testing. It distinguishes compounds from mixtures (which can have variable composition).
Safety Information
If a compound's elemental analysis shows composition different from the theoretical (law of definite proportions), the sample may be contaminated, polymorphic, or contain water of crystallisation. In pharmaceutical quality control, deviations from expected composition indicate a quality failure.
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
SynonymsProust's law, law of constant proportions, law of definite composition
Frequently Asked Questions
Also called Proust's law, it states that a pure compound always contains the same elements in the same proportions by mass regardless of the source or method of preparation. For example, water is always 88.9% oxygen and 11.1% hydrogen by mass.
The law of definite proportions underpins: the concept of empirical formulas, stoichiometric calculations, quality control of chemical products (ensuring correct composition), and pharmaceutical purity testing. It distinguishes compounds from mixtures (which can have variable composition).
If a compound's elemental analysis shows composition different from the theoretical (law of definite proportions), the sample may be contaminated, polymorphic, or contain water of crystallisation. In pharmaceutical quality control, deviations from expected composition indicate a quality failure.
Editorial standards: Chemical data is sourced from peer-reviewed literature,
CAS Registry, NIST WebBook, and PubChem. Safety information reflects guidance from OSHA, ECHA,
and IAEA. For educational purposes only — always consult official SDS documentation and qualified
professionals before handling chemicals.
