MCAT General Chemistry Question 261: Answer and Explanation

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Question: 261

3. Which one of the salts in Table 1 has the smallest value of ∆G^o_diss?

A. PbCl₂
B. MgCO₃
C. BaSO₄
D. AgCl

Correct Answer: A

Explanation:

A The important relationship between the standard state Gibbs free energy of dissolution and the solubility product constant is given in the passage:

∆G^o_diss = –RTlnK_sp

Since the question asks for an extreme, first eliminate the two choices for K_sp in Table 1 that are the middle values (choices B and C). The ln function is similar to the log function, and can be thought of in the same way when judging relative magnitudes of ∆G^o_diss in the equation above. For values of K_sp > 1, the ∆G^o value will be negative, and for values of K_sp < 1, the ∆G^o value will be positive. Therefore, the larger value of K_sp for PbCl₂ will give the smallest value for ∆G^o.

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