The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is
calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those
substances whose constituent particles are ions. For example, sodium chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be calculated as–
1 × 23 + 1 × 35.5 = 58.5 u