Question

Find the % composition of the elements in the following compounds: (1) Water (2) Sodium Sulphate

Answer 1

(i) Molar mass of water (H₂O)

= mass of 2H + mass of O

= 2 x 1 + 16 = 18g

i.e, 18g of water contains 2g of hydrogen and 16g of oxygen

∴ Percentage of hydrogen

= \(\frac{in\, water}{Molar\, mass}\) x 100

= \(\frac{2}{18}\) x 100 = \(\frac{100}{9}\) = 11.11%

∴ Percentage of oxygen

mass of oxygen

= \(\frac{in\, water}{Molar\, mass}\) x 100

= \(\frac{16}{18}\) x 100 = \(\frac{800}{9}\) = 88.89%

(b) Molar mass of sodium sulphate N₂SO₄

= 2(Na) + (S) + 4(O)

= 2(23) + 32 + 4(16)

= 46 + 32 + 64 = 142 G

142 g sodium sulphate contains 46 g sodium, 32 g sulphur and 64 g oxygen.

Percentage of sodium

= \(\frac{46}{142}\) x 100 = \(\frac{2300}{71}\) = 32.39%

Percentage of sulphur

= \(\frac{32}{142}\) x 100 = \(\frac{1600}{71}\) = 22.54%

Percentage of oxygen

= \(\frac{64}{142}\) x 100 = \(\frac{3200}{71}\) = 45.07%