MCAT General Chemistry Question 126: Answer and Explanation

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Question: 126

6. The entropy change when a solution forms can be expressed by the term ΔS°soln. When an ion dissolves and water molecules are ordered around it, the ordering would be expected to make a negative contribution to ΔS°soln. An ion that has more charge density will have a greater hydration effect, or ordering of water molecules. Based on this information, which of the following compounds will have the most negative ΔS°soln?

  • A. KCl
  • B. LiF
  • C. CaS
  • D. NaCl

Correct Answer: C

Explanation:

CaS will cause the most negative ΔS°soln because the Ca2+ and S2- ions have the highest charge density compared to the other ions. All of the other ions have charges of +1 or -1,whereas Ca2+ and S2- each have charges with a magnitude of 2.

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