CBSE Class 9 Answered

The formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the term formula unit for those substances whose constituent particles are ions.

Example:

Let us determine the formula unit mass of calcium chloride.

The molecular mass of calcium chloride (CaCl₂) is the sum of the masses of one calcium atom and two chlorine atoms.

Therefore, the molecular mass of calcium chloride (CaCl₂) = 1 × (Atomic mass of calcium) + 2 × (Atomic mass of chlorine).

                                                                                   = 1 × (40) + 2 × (35.5)

                                                                                   = 40 + 71

                                                                                   = 111 u

Therefore, the formula unit mass of calcium chloride is 111 u.