MCAT General Chemistry Question 224: Answer and Explanation

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Question: 224

2. A group of scientists is studying the dynamics of the acetic acid dissociation below and bring the process to equilibrium under standard conditions. If the scientists then add 35 g of sodium acetate to the reaction container, which of the following will be true?

CH₃COOH(aq) CH₃COO^-(aq) + H⁺ (aq)

A. Q > K_eq and the reaction will move in reverse.
B. Q < K_eq and the reaction will move forward.
C. Q > K_eq and the reaction will move forward.
D. Q < K_eq and the reaction will move in reverse.

Correct Answer: A

Explanation:

A K_eq = [products]/[reactants] when both reactant and product concentrations are those at equilibrium. Q = [products]/[reactants] regardless of whether reactant and product concentrations are those at equilibrium. The addition of sodium acetate essentially translates into the addition of acetate ion, a product in this equilibrium. As a result of such an addition, Q > K_eq and products are present in excess of equilibrium values. Le Châtelier's principle states that net reverse movement is created when the concentration of products is increased in an equilibrium system.

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