How many grams of H₂SO₄ must be added to neutralise 200 mL of 0.1 M solution of the alkaline substance formed?

Correct answer: b 1.96g

Ca(OH)₂ = 0.1 × 0.2 = 0.02 mol; the ratio is 1 : 1, so H₂SO₄ is 0.02 mol = 0.02 × 98 = 1.96 g.

Why the other options are wrong

• a 0.98g: 0.98 g is 0.01 mol, only half of what is needed. In Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O the ratio is 1 : 1, so 0.02 mol Ca(OH)₂ needs 0.02 mol H₂SO₄.
• c 4.9g: 4.9 g is 0.05 mol; a 0.1 M solution holds 0.05 mol in 500 mL, but here the volume is 200 mL, so 0.02 mol.
• d 9.8g: 9.8 g is 0.1 mol, the amount in a full litre. The solution is only 200 mL, so 0.02 mol, which is 1.96 g.