Formula unit mass is defined as the sum of the mass of all the atoms each multiplied its atomic masses that are present in the empirical formula of a compound. It is expressed in amu.
Atomic mass of calcium = 40 amu
Atomic mass of chlorine = 35.5 amu
Formula mass of CaCl2 = (1 x 40) + (2 x 35.5) = 111amu.
