Both molecular mass and formula mass signify the same thing that is the total mass of a substance. While molecular mass is used for covalent compounds, the term formula mass is used for ionic compounds.
The sum of atomic masses of all the atoms as present in the empirical or simple formula of a compound is called formula mass. The units of formula mass “atomic mass unit” (amu). For e.g NaCl= 23 + 35.5 = 58.5 amu
Similarly, the sum of atomic masses of all the atoms as present in the molecular formula of that compound is called molecular mass. For e.g. H2O=1×2+16=18 amu.