2.3.3—Terms: empirical and molecular formula

An empirical formula shows the simplest whole-number ratio of atoms or ions in a substance. A molecular formula shows the actual number of each atom in one molecule and is a whole-number multiple of the empirical formula when the substance is molecular.

Use the question being asked to choose the representation: empirical formula answers 'what is the simplest composition ratio?', whereas molecular formula answers 'how many atoms are present in one molecule?'. For an ionic compound, the formula unit gives the simplest ratio rather than a discrete molecule.

To relate the two, divide the molecular formula by the empirical formula to obtain a whole-number multiple. The molecular formula may equal the empirical formula when the simplest ratio is already the actual molecular composition; otherwise every empirical subscript is multiplied by the same integer.

Do not call an empirical formula an incomplete molecular formula, and do not change individual subscripts independently. Calculating the ratio from mass or percentage data belongs to the neighbouring calculation objective; this card establishes the meanings and relationship of the two terms.