For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the equilibrium constant Kc is 4.0 at a certain temperature. If the equilibrium concentrations are [N2] = 0.50 M and [H2] = 0.50 M, what is the equilibrium concentration of NH3?
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Correct answer
A. 0.50 M
Principle or equation
For the reaction aA + bB ⇌ cC, Kc = [C]^c / ([A]^a [B]^b). Rearrange to solve for the unknown concentration.
Why this answer is correct
Kc = [NH3]^2 / ([N2][H2]^3). Substitute given values: 4.0 = [NH3]^2 / (0.50 × (0.50)^3) = [NH3]^2 / (0.50 × 0.125) = [NH3]^2 / 0.0625. Therefore [NH3]^2 = 4.0 × 0.0625 = 0.25, so [NH3] = 0.50 M.
Example
For 2A ⇌ B, Kc = [B]/[A]^2. If Kc = 2 and [A] = 0.10 M, then [B] = 2 × (0.10)^2 = 0.020 M.
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