One thing that always puzzled me on the Nerst equation is that you may mix the units of concentration and pressure. This, however, seems to be rather arbitrary. How can using the units of mol/L and atm in the same equation result in accurate results? It seems to be rather arbitrary! Particularly, when you use PV = nRT and calculate what the "concentration" of gas molecules in a sample of gas at 1atm and 298K, one does not get 1 mol/L. I could agree if people used something on the lines of k*(pressure in atmospheres) in Nernst equation, where k is a correction factor. But setting k = 1 (which is what is done in chemistry textbooks) seems to be arbitrary, requiring a tremendous coincidence between the units mol/L and atm! Can anyone explain me why mixing units in the Nernst equation is fine?