The term molecular mass is used for molecular compounds. Tonic compounds consist of arrays of oppositely charged ions rather than separate molecules. So, we represent an ionic compound by its formula unit. A formula unit indicates the simplest ratio between cations and anions in an ionic compound.

Example:

For example, the common salt consists of Na⁺ and Cl^- ions. It has one Na⁺ ion for every Cl^- ion. So, formula unit for common salt is NaCl.