MCAT General Chemistry Question 226: Answer and Explanation

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Question: 226

4. Na₂SO₄ is soluble in water. If NaCl(s) is added to a solution of Na₂SO₄(aq) so that the concentration of Na⁺ doubles, then the:

A. solubility constant of Na₂SO₄ increases while that of NaCl decreases.
B. solubility constants of Na₂SO₄ and NaCl both decrease.
C. solubility of Na₂SO₄ and NaCl both decrease.
D. solubility of Na₂SO₄ decreases while that of NaCl increases.

Correct Answer: C

Explanation:

C Solubility constants, like all equilibrium constants, are functions of temperature only. This eliminates choices A and B. Given the equilibria:

Na₂SO₄(s) 2 Na⁺ (aq) + SO^2-₄(aq)
NaCl(s) Na⁺ (aq) + Cl^- (aq)

Na⁺ is a common ion to both systems. Increasing Na⁺ concentration will decrease the solubility of both salts, eliminating choice D and making choice C the best answer.

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