Calculate the percentage by weight of :
(a) C in carbon dioxide
(b) Na in sodium carbonate
(c) Al in aluminium nitride.
[C = 12, O = 16, H = 1, Na = 23, Al = 27, N = 14]

(a) Molecular weight of carbon dioxide CO₂ = 12 + (2 × 16) = 12 + 32 = 44

44 g of CO₂ contains 12 g of carbon

∴ 100 g of CO₂ contains 12\over44 × 100 = 27.27 % of carbon

Hence, the percentage by weight of C in CO₂ is 27.3%

(b) Molecular weight of sodium carbonate Na₂CO₃

= (2 × 23) + 12 + (3 × 16)

= 46 + 12 + 48

= 106

106 g of sodium carbonate Na₂CO₃ contains 46 g of sodium

∴ 100 g of Na₂CO₃ contains 46\over106 × 100 = 43.39 % ≈ 43.4% of sodium.

Hence, percentage by weight of sodium in sodium carbonate is 43.4%

(c) Molecular weight of Aluminium Nitride = AlN

= 27 + 14 = 41

41 g of Aluminium Nitride contains 27 g of Aluminium

∴ 100 g of Aluminium Nitride contains 27\over41 × 100

= 65.85% of Aluminium

Hence, the percentage by weight of Aluminium in Aluminium Nitride is 65.85%.