Solution: From formula
Mass % of an element = Molar mass of component/Molar mass of compound ×100
The molecular formula of compound (Na2SO4) is
= [(2 × 22.99) + (32.06) + 4 (16.00)]
= 142.04 g
Molar mass of 2Na = 2 × 22.99 = 45.98 amu
Molar mass of 4O = 4 × 16.00 = 64.00 amu
Now put the values in above formula
Mass percent of sodium = 45.98 ×100 /142.04 = 32.37%
Mass percent of sulphur = 32.06 ×100 /142.04 = 22.57%
Mass percent of oxygen = 64.00 ×100 /142.04 = 45.06%