For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), the equilibrium constant Kc is 4.0 at a certain temperature. If the equilibrium concentrations are [SO2] = 0.40 M and [O2] = 0.20 M, what is the equilibrium concentration of SO3?
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Correct answer
C. 0.40 M
Principle or equation
The equilibrium constant expression for the reaction aA + bB ⇌ cC is Kc = [C]^c / ([A]^a [B]^b). For this reaction, Kc = [SO3]^2 / ([SO2]^2 [O2]). Rearrange to solve for [SO3].
Why this answer is correct
The equilibrium expression is Kc = [SO3]^2 / ([SO2]^2 [O2]). Substitute: 4.0 = [SO3]^2 / (0.40^2 × 0.25). Compute denominator: 0.16 × 0.25 = 0.04. Then [SO3]^2 = 4.0 × 0.04 = 0.16, so [SO3] = 0.40 M.
Example
For a reaction with Kc = 1.0, [A] = 0.10 M, [B] = 0.10 M, and the equation A + B ⇌ C, then [C] = Kc × [A][B] = 1.0 × 0.01 = 0.01 M.
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