1.3 Elemental Composition of Pure Substances

Explain the quantitative relationship between the elemental composition by mass and the empirical formula of a pure substance.

• Some pure substances are composed of individual molecules, while others consist of atoms or ions held together in fixed proportions as described by a formula unit.
• According to the law of definite proportions, the ratio of the masses of the constituent elements in any pure sample of that compound is always the same.
• The chemical formula that lists the lowest whole number ratio of atoms of the elements in a compound is the empirical formula.