For the reaction 2NO(g) + O2(g) ⇌ 2NO2(g), the equilibrium constant Kc is 8.0 at a certain temperature. If the equilibrium concentrations are [NO] = 0.20 M and [O2] = 0.10 M, what is the equilibrium concentration of NO2?
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Correct answer
B. 0.18 M
Principle or equation
For the reaction aA + bB ⇌ cC, the equilibrium constant expression is Kc = [C]^c / ([A]^a [B]^b).
Why this answer is correct
Write the equilibrium expression: Kc = [NO2]^2 / ([NO]^2 [O2]). Substitute the given values: 8.0 = [NO2]^2 / (0.20^2 × 0.10). Compute denominator: 0.04 × 0.10 = 0.0040. So [NO2]^2 = 8.0 × 0.0040 = 0.032. Taking the square root gives [NO2] = 0.18 M (rounded to two significant figures).
Example
For N2(g) + 3H2(g) ⇌ 2NH3(g), if Kc = 4.0, [N2] = 0.50 M, [H2] = 0.20 M, then 4.0 = [NH3]^2 / (0.50 × 0.20^3) = [NH3]^2 / 0.0040, so [NH3]^2 = 0.016, [NH3] = 0.13 M.
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