Challenge Your Exam Readiness With Our Free ACS General Chemistry Practice Questions

Set up an ICE table in terms of partial pressures. Let x be the dissociation of N₂O₄: P(N₂O₄) = 1.00 − x, P(NO₂) = 2x. The equilibrium constant in pressure terms is Kp = (P_NO₂)² / P_N₂O₄ = (2x)² / (1 − x) = 4x²/(1 − x) = 0.15. Solving gives x ≈ 0.176, so P(NO₂) = 2x ≈ 0.352 atm (choice C). This aligns with ACS equilibrium competencies: translating a chemical equation into an equilibrium expression, constructing an ICE table, and solving a quadratic while checking that the result is physically reasonable (x < initial pressure). Distractors often correspond to using concentration-based Kc without converting, plugging in initial conditions instead of equilibrium values, or neglecting the stoichiometric factor of 2 for NO₂. Because total moles of gas increase upon dissociation, the formation of NO₂ is favored by low total pressure; however, numerical evaluation via Kp provides the exact equilibrium composition, a key quantitative skill on the exam.