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title: "At a certain temperature, the equilibrium constant \\(K_c\\) for the reaction represented below is \\(15\\).  \\[\\text{PCl}_3(g) + \\text{Cl}_2(g) \\rightleftharpoons \\text{PCl}_5(g)\\]  A student injects \\(0.20\\text{ mol}\\) of \\(\\text{PCl}_3(g)\\), \\(0.20\\text{ mol}\\) of \\(\\text{Cl}_2(g)\\), and \\(0.40\\text{ mol}\\) of \\(\\text{PCl}_5(g)\\) into an evacuated, rigid \\(2.0\\text{ L}\\) container at this temperature. Which of the following correctly predicts how the concentration of \\(\\text{PCl}_5(g)\\) will change as the system approaches equilibrium, and provides the correct justification?"
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url: "https://nerd-notes.com/ubq/121470/"
date_modified: "2026-08-23T05:04:44+00:00"
---

# At a certain temperature, the equilibrium constant \(K_c\) for the reaction represented below is \(15\).

\[\text{PCl}_3(g) + \text{Cl}_2(g) \rightleftharpoons \text{PCl}_5(g)\]

A student injects \(0.20\text{ mol}\) of \(\text{PCl}_3(g)\), \(0.20\text{ mol}\) of \(\text{Cl}_2(g)\), and \(0.40\text{ mol}\) of \(\text{PCl}_5(g)\) into an evacuated, rigid \(2.0\text{ L}\) container at this temperature. Which of the following correctly predicts how the concentration of \(\text{PCl}_5(g)\) will change as the system approaches equilibrium, and provides the correct justification?

At a certain temperature, the equilibrium constant \(K_c\) for the reaction represented below is \(15\).

\[\text{PCl}_3(g) + \text{Cl}_2(g) \rightleftharpoons \text{PCl}_5(g)\]

A student injects \(0.20\text{ mol}\) of \(\text{PCl}_3(g)\), \(0.20\text{ mol}\) of \(\text{Cl}_2(g)\), and \(0.40\text{ mol}\) of \(\text{PCl}_5(g)\) into an evacuated, rigid \(2.0\text{ L}\) container at this temperature. Which of the following correctly predicts how the concentration of \(\text{PCl}_5(g)\) will change as the system approaches equilibrium, and provides the correct justification?

- **A.** The concentration of \(\text{PCl}_5(g)\) will increase because \(Q_c < K_c\), requiring the net forward reaction to occur to establish equilibrium.
- **B.** The concentration of \(\text{PCl}_5(g)\) will increase because \(Q_c > K_c\), which favors the formation of additional products to establish equilibrium.
- **C.** The concentration of \(\text{PCl}_5(g)\) will decrease because \(Q_c > K_c\), requiring the net reverse reaction to occur to establish equilibrium.
- **D.** The concentration of \(\text{PCl}_5(g)\) will decrease because \(Q_c < K_c\), requiring the consumption of reactants to establish equilibrium.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/121470/*