Formula mass

 Formula mass

The average mass of a formula unit of an ionic compound in atomic mass unit is called its formula mass.
It is applicable to ionic substance .Formula unit of NaCl and formula mass is 23 + 55 = 58.5amu according to crystal structure.

Formula unit

                       A formula unit as the implies , is one unit representing simple ratio between positive and negative charge.

For example 

                     The formula unit of NaCl have one Na and and one Cl.