Gram formula mass:

Formula mass of a substance expressed in gram is called gram formula mass.

An ionic compound is represented by the formula unit that represents the simplest ratio between the ions of a compound.

Examples:

NACI, KCI, CuSO₄, etc

Formula mass of NaCl = 23 + 35.5

= 58.5 amu

Therefore, gram formula mass of NaCl = 58.5g

Formula mass of KCl = 39 + 35.5

= 74.5 amu

So, gram molecular mass of KCl = 74.5g