Define formula mass.
The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass.
- 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
- Formula mass of Mg (OH)2 = 24 + 16 x 2 + 1 x 2
= 24 + 32 + 2
= 58 amu
The term molecular mass is used for molecular compounds. Whereas, the term formula mass is used for ionic compounds.