For the reaction 2A(g) + B(g) ⇌ 2C(g), the equilibrium constant Kc is 0.25 at a certain temperature. If the equilibrium concentrations are [A] = 0.40 M and [B] = 0.20 M, what is the equilibrium concentration of C?
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Correct answer
C. 0.080 M
Principle or equation
For the reaction aA + bB ⇌ cC, Kc = [C]^c / ([A]^a [B]^b). Rearrange to solve for [C].
Why this answer is correct
Kc = [C]^2 / ([A]^2 [B]). Plug in: 0.25 = [C]^2 / (0.40^2 × 0.20) = [C]^2 / 0.032. So [C]^2 = 0.25 × 0.032 = 0.0080. [C] = sqrt(0.0080) = 0.0894 M, approximately 0.080 M.
Example
If Kc = 1.0, [A] = 0.10, [B] = 0.10, then [C] = sqrt(1.0 × 0.10^2 × 0.10) = sqrt(0.001) = 0.032 M.
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