The sum of atomic masses of all the atoms present in one molecule of a molecular substance, is its molecular mass. For example, molecular mass of chlorine (Cl2 ) is 71.0 amu, of water (H2O) is 18 amu and that of carbon oxide (CO2) is 44 amu.
Formula mass; The sum of atomic masses of all the atoms present in one formula unit of a substance is called formula mass. Some ionic compounds that form three dimensional solid crystals, are represented by their formula units. Formula mass in such cases is the sum of atomic masses of all the ions present in a formula unit. For example, formula mass of sodium chloride is 58.5 amu and that of CaCO3 is 100 amu.