Formula mass:

The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass.

Examples:

1. Formula mass of NaCl = 1 x Atomic mass of Na + 1 x Atomic mass of CI

= 1 x 23 + 1 x 35.5

= 58.5 amu

2. Formula mass of Mg (OH)₂ = 24 + 16 x 2 + 1 x 2

= 24 + 32 + 2

= 58 amu

Note:

The term molecular mass is used for molecular compounds. Whereas, the term formula mass is used for ionic compounds.