Question

(i) What do the following symbols / formulae stand for: 

(a) 2O      (b) O₂       (c) O₃       (d) H₂O 

(ii) Give the chemical formula of the following compounds: 

(a) Potassium carbonate (b) Calcium chloride 

(iii) Calculate the formula unit mass of Al₂(SO₄)₃. 

(Given: Atomic mass of Al = 27 u, S = 32 u, O = 16 u) 

Answer 1

(i) (a) 2O = 2 atoms of oxygen 

(b) O₂ = Diatomic oxygen 

(c) O₃ = Triatomic oxygen / ozone 

(d) H₂O=Two atoms of hydrogen and one atom of oxygen construct one molecule of water (H₂O) 

(ii) Potassium carbonate - K₂CO₃ 

Calcium chloride - CaCl₂ 

(iii) Formula unit mass of any compound is the sum of atomic masses of all atoms in the formula of that compound. 

Formula unit mass of Al₂(SO₄)₃ 

Atomic mass of Al = 27 × 2 = 54 u 

Atomic mass of S = 32 × 3 = 96 u 

Atomic mass of O = 16 × 12 = 192 u 

Formula unit mass = 54 + 96 + 192 = 342 u. 

Hence, Formula unit mass of Al₂(SO₄)₃ is 342.

Answer 2

2O represent 2 separate atom while O₂ represent 1 molecules of oxygen gas containing 2 atoms