What is the effect of adding an inert gas to a gaseous equilibrium at constant volume?

• A. Shift toward side with more moles
• B. Shift toward side with fewer moles
• C. No change in the position of equilibrium
• D. Increase the equilibrium constant

Answer: (C)

Adding an inert gas at constant volume changes the total pressure but not the amounts of the reacting gases. Since the volume and temperature are fixed, the partial pressures of each reacting species are P_i = n_i RT / V, so they remain unchanged when an inert gas is added. The equilibrium position is determined by the partial pressures that appear in the equilibrium constant expression; with those partial pressures staying the same, the reaction quotient equals the constant and no shift is required. The only change is the total pressure rises, not the balance between reactants and products. If temperature or the participating gas amounts changed, or if the volume were allowed to change, the situation could be different, but under these conditions there is no change in the position of equilibrium.