In irreversible P-V expansion of gas, workdone by the gas(system) is often expressed in terms of the external pressure without considering the internal pressure of the gas.Why is this so? Can we not determine the work in terms of internal pressure of the system? Please explain about it more elaborately.
                   

Actually in chemistry we consider the factors that affect the internal energy of system ( its quite different from physics where we consider factors that affect energy of surroundings). Since it is the external pressure that increase or decrease the internal energy of the system thus we take work sign conventions ( +ve if gas expands due to decrease in internal energy by greater external pressure and -ve when gas contracts) and P-V diagrams according to external pressure and not internal pressure.