if we choose the 1 atm ideal gas standard state for gas B,P=1.
Thus,it is customary to express activities of gaseous species iin terms of the pressure symbol P.it has to be remembered that P1 in the formulation of K is in fact P/P1 and hence has the same numerical value as the pressure in atm unit of the species i only if the 1 atm ideal gas standard state is chosenfor species i.