What is the difference between molecular mass and formula mass?

For term formula mass is used, while for molecular term molecular mass is used.

Sum of atomic masses of all the atoms as present in empirical or simple formula of a compound is called formula mass. As e.g. ##NaCl##, 23 + 35.5 = 58.5 ##"amu"##.

Similarly sum of atomic masses of all the atoms as present in the molecular formula of that compound is called molecular mass. As e.g. ##H_2O##, is 1 x 2 + 16 = 18 ##"amu"##.