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Law of Definite Proportions

The law of definite proportions, first defined by Joseph Louis Proust, states that in a pure compound, the elements combine in definite proportions to one another by mass. In other words, we know that water is always composed of two hydrogen and one oxygen atom, typically about 88.8% oxygen to about 11.2% hydrogen by mass (heavy water, with a different oxygen isotope, changes this proportion somewhat, but also in a predictable manner.)

Not all compounds composed of the same element will combine in the same definite proportions. Hydrocarbons, for instance, go through many hundreds of permutations, each with somewhat different properties, and may have from one to hundreds of carbon atoms combined with hydrogen atoms in differing proportions. And hydrogen peroxide is made up of oxygen and hydrogen (as is water), but with a different proportion of hydrogen to oxygen than water.

There is one class of substances called non-stoichiometric compounds or Berthollides that are an exception to this law. The ratio between elements in these compounds can vary within certain limits. Rust, or ferrous oxide, is one of the berthollides. Its primary formula is simply FeO, but can vary significantly.

Web Resources On Law of Definite Proportions

The Law of Definite Proportions: Proust 1797
Law of Definite Proportions Reacting Ratios

Book Resources On Law of Definite Proportions

Basic Physics : A Self-Teaching Guide by Karl F. Kuhn
Chemistry: Matter and Its Changes by Brady & Senese

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