A given compound always contains the same elements in the same mass proportions regardless of source.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
Also called the law of constant composition, it states that a given chemical compound always contains the same elements in the same mass proportions. Joseph Proust established it by analysing samples from many sources around 1800. The law supports the idea that compounds are made of atoms joined in fixed ratios, because the mass of each element per atom is fixed. It allows chemists to identify a compound by its percentage composition and to calculate its empirical formula. Mixtures, by contrast, can have any composition.
Key ideas: A given compound always contains the same elements in the same mass ratio (Proust), whatever its source or method of preparation. Water is always about 1 part hydrogen to 8 parts oxygen by mass (2.016 : 16.00). Use it to check unknown samples: compute the mass percent and compare with the formula.
Common mistakes and connections: Students apply it to mixtures, which can have any ratio. Another error is quoting it as a ratio of atoms rather than masses, forgetting atomic masses matter. Nonstoichiometric compounds such as FeO (actually Fe0.95O) are real exceptions. It pairs with multiple proportions as evidence for Dalton's theory.
Any pure sample of water, from any source, is 88.8 percent oxygen and 11.2 percent hydrogen by mass, which illustrates constant composition. The interactive lesson shows this as a live animation or chart.
Water is always about 11.2 percent hydrogen and 88.8 percent oxygen by mass, whether it comes from a river, from rain or from a laboratory synthesis.
A given compound always contains the same elements in the same mass proportions regardless of source.
Law of Definite Proportions is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Physical Change, Chemical Change, Law of Conservation of Mass.
The next topics in Atomic Structure & Periodicity are Law of Multiple Proportions and Dalton's Atomic Theory.
Yes. The interactive lesson includes a self-check question on Law of Definite Proportions, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Law of Definite Proportions is topic 10 of 115 in Atomic Structure & Periodicity.