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?
At constant temperature, only changes to the concentration (technically activity) of a reactant or product, shift the equilibrium.
Adding inert gas does not change the concentration of any reactant or product, so it does not affect equilibrium.
The gas added works in both directions and the fraction of pressure is the same becaussed the total pressure increses but the inert gas do not affect the features of the others and exerts the same effect. If the number of molecules do not change, the concentration remains constant, and the principle of Le chatelier sais that in case of increase the pressure the reaction take the direction where there are less molecules because the pressure force them to agregate.