just kidding. You actually use the Ideal gas equation and (think about the assumptions made for the ideal gas law) calculate delta(V). Then, think about what Cj means
This problem is a bit complicated, but I remember it from my days as an undergraduate. You have to use a Laplacian transform of the Ideal Gas equation (assuming that T is high enough at 1atm to support this assumption). This should give you an equation for concentration in the form of a...