$N X_{2} (g) + 3 H X_{2} (g) 2 NH X_{3} (g)$ is at equilibrium in a cylinder with a movable piston. Argon is added at constant temperature and constant total pressure (so the volume increases). Which statement describes the situation just after the addition?

Answer: C

Increasing the volume by a factor

f

divides every concentration by

f

Q_{c} = [NH X_{3}]^{2} / ([N X_{2}] [H X_{2}]^{3})

changes by

f^{- 2} / f^{- 4} = f^{2}

, so

Q_{c} = f^{2} K_{c} > K_{c}

. The reaction therefore goes backward, toward the side with more moles of gas.