Dynamic equilibrium - effect of adding inert gas [duplicate]

Well consider the reaction $A + B ⇌ C$ Adding inert gas to a container where this reaction is taking place will increase the pressure of the system. In accordance with Le Chatelier's Principle, the equilibrium should shift in the direction which opposes this change in pressure, i.e. in the direction where fewer number of molecules are created. In this specific reaction, it happens to be in the forward reaction. However, I have been taught that adding an inert gas at constant volume has no effect on the equilibrium. Why is this?

• In the case of, say, the equilibrium partial pressure of water vapor over water, the pressure of inert gas over the water is ignored. This is because water is (pretty much) incompressible, so the inert gas pressure has no effect and Le Chatelier's principle results in no shift in equilibrium for any reasonable lab pressures. In many intro courses simple absolutes are presented - in the context of the introductory material they are fine, but the principles break down eventually, which is then the basis of the next courses... – Jon Custer Jan 20 '15 at 17:23
• @JonCuster Well that may be applicable for the certain example that you have provided, but definitely not for all reactions. There are many equilibrium reactions which contain all reactants and products in gas phase. – Gummy bears Jan 21 '15 at 15:40