Law of Definite Proportions
| Also Known As | Proust's law, law of constant proportions, law of definite composition |
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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
Uses & Applications
Safety Information
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
Key Facts
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.