Exemplar Problems

Problem 5 : At a certain temperature, the equilibrium constant Kc for the reaction:

PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) is 0.025. If the initial concentration of PCl₅ is 0.1 M, calculate the equilibrium concentrations of PCl₃ and Cl₂.

Solution :

Let’s denote the initial concentration of PCl₅ as [PCl₅]₀, and the equilibrium concentrations of PCl₃ and Cl₂ as [PCl₃] and [Cl₂], respectively. We are given: [PCl₅]₀ = 0.1 M Kc = 0.025

Using the balanced equation, we know that at equilibrium: [PCl₅] = [PCl₅]₀ - x [PCl₃] = [PCl₃]₀ + x [Cl₂] = [Cl₂]₀ + x

Now, we can write the expression for Kc: Kc = ([PCl₃] * [Cl₂]) / [PCl₅]

Substitute the expressions for [PCl₃], [Cl₂], and [PCl₅]: 0.025 = ([PCl₃]₀ + x) * ([Cl₂]₀ + x) / (0.1 - x)

Now, solve for x. You can use numerical methods or a calculator to find the value of x.