Elemental composition of pure substances

Pure substances are composed either of discrete molecules (as in molecular compounds like H₂O) or of atoms and ions held in fixed proportions described by a formula unit (as in ionic compounds like NaCl and network solids like SiO₂).

Key Definition The law of definite proportions guarantees that every pure sample of a given compound has the same mass ratio of its constituent elements.

This invariance arises because each formula unit or molecule contains a fixed number of each type of atom, and each atom has a characteristic molar mass. The constancy of mass composition is the foundation for all quantitative analysis of chemical formulas.

The empirical formula

• Expresses the lowest whole-number ratio of atoms of each element in a compound
• Is determined experimentally by converting mass composition data to moles and then simplifying the mole ratio to the smallest whole numbers

Because multiple compounds can share the same empirical formula while having different molecular formulas, the empirical formula alone does not uniquely identify a molecular substance — additional information, such as the molar mass, is needed to determine the molecular formula. Mastery of the percent-composition-to-empirical-formula conversion is essential, as it appears frequently on both the multiple-choice and free-response sections of the AP exam.