MCAT General Chemistry Question 134: Answer and Explanation

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Question: 134

14. A saturated solution of cobalt(III) hydroxide (K_sp = 1.6 × 10^-44) is added to a saturated solution of thallium(III) hydroxide (K_sp = 6.3 × 10^-46). What is likely to occur?

A. Both cobalt(III) hydroxide and thallium(III) hydroxide remain stable in solution.
B. Some cobalt(III) hydroxide precipitates and thallium(III) hydroxide remains stable in solution.
C. Some thallium(III) hydroxide precipitates and cobalt(III) hydroxide remains stable in solution.
D. Some of both cobalt(III) hydroxide and thallium(III) hydroxide precipitate.

Correct Answer: C

Explanation:

Thallium(III) hydroxide has a lower K_sp value than cobalt(III) hydroxide. It is important to note that one can assume the molar solubility of thallium(III) hydroxide is lower than cobalt(III) hydroxide only because both salts have a formula MX₃ (one of one particle, three of another). When the solutions are mixed, [OH^-] is at saturation levels in the cobalt solution—which is higher than saturation levels in the thallium solution. Therefore, the ion product for thallium(III) hydroxide is higher than its solubility product constant, and the system will shift left to form solid thallium(III) hydroxide, which precipitates.

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