Question

Calculate the molar mass of the following compounds:

(i) MgO 

(ii) Na₂CO₃ 

(iii) H₃PO₄ 

(iv) Ca(OH)₂ 

(v) Al₂(SO₄)₃

Answer 1

i. MgO: 

Molar mass of Mg + 2 × Molar mass of oxygen

⇒ 24u + 2×16 

⇒ 24 + 32 

⇒ 56g/mol

Thus, the molar mass of MgO is 56g/mol.

ii. Na₂CO₃ : 

2 × Molar mass of Na + Molar mass of carbon + 3×molar mass of oxygen 

⇒ 2 × 23 + 12 + 3 × 16 

⇒ 46 + 12 + 48 

⇒ 106g/mol 

Thus, the molar mass of Na₂CO₃ is 106g/mol.

 iii. H₃PO₄ :

3 × Molar mass of hydrogen + Molar mass of phosphorus + 4×molar mass of oxygen 

⇒ 3 × 1 + 31 + 4 × 16 

⇒ 3 + 31 + 64 

⇒ 98g/mol 

Thus, the molar mass of H₃PO₄ is 98g/mol. 

iv. Ca(OH)₂ 

Molar mass of calcium + 2 × Molar mass of oxygen + 2×molar mass of hydrogen 

⇒ 40 + 2 × 16 + 2 × 1 

⇒ 40 + 32 + 2 

⇒ 74g/mol 

Thus, the molar mass of Ca(OH)₂ is 74g/mol. 

v. Al₂(SO₄)₃ 

2 × Molar mass of aluminium + 3 × Molar mass of sulphur + 12×molar mass of oxygen 

⇒ 2 × 27 + 3 × 32 + 12 × 16 

⇒ 54 + 96 + 192 

⇒ 342g/mol 

Thus, the molar mass of Al₂(SO₄)is 342g/mol