MCAT General Chemistry Question 97: Answer and Explanation

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Question: 97

7. For a certain chemical process, What is the equilibrium constant K_eq for this reaction? (Note: )

A. K_eq = 1.0
B. K_eq = 7.4
C. K_eq = 8.9
D. K_eq = 10

Correct Answer: B

Explanation:

Solve this question using the equation ΔG^°_rxn = -RT ln K_eq. ΔG_rxn^° is

R is

and T = 298 K because the reaction is occurring under standard conditions. Because ΔG^°_rxn uses kilojoules in its units and R uses joules, one will have to be converted. Plugging into the equation, we get:

If ln K_eq = 2, then K_eq = e². e is approximately 2.7, so we're looking for a number between 4 (2²) and 9 (3²). Both choices (B) and (C) fit these criteria; however, 8.9 is very close to 9, so we can assume that its square root is very, very close to 3. The answer choice should be a bit smaller, so choice (B), 7.4, is correct.

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