Relative Formula Mass - GCSE Chemistry Definition

The relative formula mass, written as M_r, is the sum of the relative atomic masses (A_r) of all the atoms in the formula unit of a substance. It is mainly used for ionic compounds or substances with giant structures, where individual molecules do not exist.

For example, in sodium chloride (NaCl):

M_r = (1 × Na) + (1 × Cl) = 23 + 35.5 = 58.5

Although relative molecular mass is technically used for simple molecules (like H₂O or CO₂), at GCSE level, students are not expected to distinguish between these terms. The term relative formula mass is generally used for all types of substances in calculations.

M_r is important in GCSE Chemistry for working out mole calculations, reacting masses, and chemical formulas.