Question

1. An analysis of equal volumes of water samples collected from different sources showed the following

Water sample source	Analysis
Natural springs	pure
Sea water	0.30 M NaCl
Lake water	0.11 M Na3PO4
Waste water treatment plant	0.60 M CaSO4

Which of the water samples will NaOH be least soluble in? Explain the choice of your answer.

Answer 1

Correct option: Lake water

Lake water sample contains 0.11 M Na₃PO₄

Na₃PO₄ (aq) $\rightleftharpoons$ 3 Na⁺ (aq) + PO₄^3- (aq)

0.11 M Na₃PO₄ in water will give 0.11 x 3 = 0.33 M Na⁺ ions

Sea water sample contains 0.30 M NaCl

NaCl (aq) $\rightleftharpoons$ Na⁺ (aq) + Cl^- (aq)

0.30 M NaCl in water will give 0.30 M Na⁺ ions.

Waste water treatment plant sample will not produce any common ion (Na⁺) in water.

NaOH dissolves in water to give Na⁺ and OH^- ions.

NaOH (aq) $\rightleftharpoons$ Na⁺ (aq) + OH^- (aq)

The common ion effect due to the same cation (Na⁺) will suppress the solubility of NaOH in lake water sample more than any other sample source mentioned in the problem.   Therefore, NaOH will be least soluble in lake water sample.