Law of Definite Proportions Definition

Elements by Mass in a Compound

Scientist holding model of a compound

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The law of definite proportions, together with the law of multiple proportions, forms the basis for the study of stoichiometry in chemistry. The law of definite proportions is also known as Proust's law or the law of constant composition.

Law of Definite Proportions Definition

The law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. The mass ratio of elements is fixed no matter where the elements came from, how the compound is prepared or any other factor. Essentially, the law is based on the fact that an atom of a particular element is the same as any other atom of that element. So, an atom of oxygen is the same, whether it comes from silica or oxygen in air.

The Law of Constant Composition is an equivalent law, which states each sample of a compound has the same composition of elements by mass.

Law of Definition Proportions Example

The law of definite proportions says water will always contain 1/9 hydrogen and 8/9 oxygen by mass.

The sodium and chlorine in table salt combine according to the rule in NaCl. The atomic weight of sodium is about 23 and that of chlorine is about 35, so from the law one may conclude dissociating 58 grams of NaCl would produce about 23 g of sodium and 35 g of chlorine.

History of the Law of Definite Proportions

Although the law of definite proportions may seem obvious to a modern chemist, the manner in which elements combine was not obvious in the early days of chemistry through the end of the 18th century. Joseph Priestly and Antoine Lavoisier proposed the law based on the study of combustion. They noted metals always combine with two proportions of oxygen. As we know today, oxygen in the air is a gas consisting of two atoms, O2.

The law was hotly disputed when it was proposed. Claude Louis Berthollet was an opponent, arguing elements could combine in any proportion to form compounds. It wasn't until John Dalton's atomic theory explained the nature of atoms that the law of definite proportions became accepted.

Exceptions to the Law of Definite Proportions

Although the law of definite proportions is useful in chemistry, there are exceptions to the rule. Some compounds are non-stoichiometric in nature, meaning their elemental composition varies from one sample to another. For example, wustite is a type of iron oxide with an elemental composition varying between 0.83 and 0.95 iron atoms for each oxygen atom (23%-25% oxygen by mass). It ideal formula is FeO, but the crystal structure is such that there are variations. The formula is written Fe0.95O.

Also, the isotopic composition of an element sample varies according to its source. This means the mass of a pure stoichiometric compound will be slightly different depending on its origin.

Polymers also vary in element composition by mass, although they are not considered true chemical compounds in the strictest chemical sense.