Formality (F) - The number of gram formula weights of solute dissolved in a liter of solution is called Formality (F) of that solution.

F = [Number Of Gram Formula Weights Of Solute/Volume Of Solution (mL)] × 1000

F = [Amount Of Solute In Grams/Gram Formula Mass Of Substance × Volume Of Solution (mL)] x1000

F = (Wₐ/Fₐ) × (1000/Vₘₗ)

Where F = gram formula weight of solute

Ionic compounds such as CuSO₄, NaCl, KNO₃ etc. are not in the form of molecules.

Therefore, on the basis of their formula unit, the formula weight is used in place of their molecular mass.

Hence, the formula weight of NaCl is 58.5 and not the molecular weight is 58.5.

But most concentration of all these compounds expressed by writing molarity instead of formality.

When formula weight and molecular weight are same, there is no difference between molarity and formality.

When some solutes are dissolved in a solvent, they get polymerised. As a result of which their molecules become secondary. (Dimer) or becomes Trimer.

In that case the formula weight becomes two or three times from molecular weight.

For example When benzoic acid is dissolved in benzene, it forms second atom (CH₅COOH)₂.

The molecular weight of bezoic acid is 122, while the solution formula mass is = 244.