Question

(a) Define 'formula unit' of an ionic compound. What is the formula unit of (i) sodium chloride, and (ii) magnesium chloride ?
(b) Calculate he formula masses of the following compounds :
(i) Calcium chloride
(ii) Sodium carbonate
(Given : Atomic masses : Ca = 40 u ; Cl = 35.5 u ; Na = 23 u ; C = 12 u ; O = 16 u)

Answer 1

(a) Formula unit of an ionic compound refers to the empirical formula of an ionic or covalent molecule. It is the lowest whole number ratio of ions that make up the electrically neutral ionic compound. Formula unit represents the smallest unit of a compound.
The formula unit of sodium chloride is NaCl.
The formula unit of magnesium chloride is MgCl₂.

(b) (i) Formula mass of calcium chloride = mass of one Ca atom + mass of two Cl atoms = 40 + 2 × 35.5 = 111
Thus, the formula mass of calcium chloride is 111 u.
(ii) Formula mass of sodium carbonate = mass of two Na atoms + mass of one C atom + mass of three O atoms = 2 × 23 + 12 + 3 × 16 = 106
Thus, the formula unit of sodium carbonate is 106 u.