Law of definite proportions

One of the fundamental observations of modern chemistry, the law of definite proportions states that, in a pure compound, the elements combine in definite proportion to each other.

This law is sometimes stated a little differently and called the law of constant composition.

With the law of multiple proportions, this forms the basis of stoichiometry.

For example, oxygen makes up ⁸/₉ of the mass of pure water, while hydrogen makes up the remaining ¹/₉.

Given our current understanding of atomic structure and atomic mass, as represented in the periodic table of the elements, we can understand this law (and it's limitations) through this example more fully by noting that the molecular mass of water (H₂O) is 18. The oxygen atom in water has a mass of 16, so that oxygen's proportion of water's mass is ¹⁶/₁₈, eg ⁸/₉ as written more simply above. Similarly with the two hydrogen atoms, which have a mass of 1 apiece, constituting ²/₁₈, or ¹/₉, of each water molecule.

The law of definite proportions may seem a trivial exercise in simple arithmetic, given the periodic table and an understanding of atomic and molecular structure and composition and an algorithm for calculating molecular mass from formulas.

One should be cautious before making such a judgment, however, to understand that this law was formulated in the 18th century, preceding the development of the periodic chart and the understanding that it embodies, by a full century and thus contributing to its formation. The table encompasses understanding developed through the law, rather than the other way around.

The law is still used in practice in interpreting the results from that subfield of analytical chemistry known as elemental analysis[?].