Define 'formula mass' of a compound.

The formula mass of a substance is defined as the sum of the atomic masses of constituent atoms in an ionic compound. This is generally used for ionic compounds which do not contain discrete molecules, but ions as their constituent units. We can say that it is the same as the molecular mass of a compound, the only difference being that formula mass is used for ionic compounds and molecular mass is used for molecules. For example, in sodium chloride, the sodium and chloride ions are arranged in a three dimensional structure, that is there are two different entities and not a single entity (like in a molecule). Therefore the formula mass of sodium chloride will simply be

= mass of sodium atom + mass of chlorine atom

= (23 + 35.5) u

= 58.5 u