Relative formula mass

Relative formula mass (Mr) is the sum of the relative atomic mass of all the atoms present in the formula of a substance. Covalent substances are molecules , so their relative formula mass is often referred to as relative molecular mass.

Example:

Calcium hydroxide, Ca(OH)2 consists of one calcium atom (Ar = 40), two oxygen atoms (Ar = 16) and two hydrogen atoms (Ar = 1).

Mr of Ca(OH)2 = 40 + 16 + 16 + 1 + 1 = 74

or

Mr of Ca(OH)2 = 40 + 2(16 + 1) = 74