I think I actually have solved it. I was right with the PV=nkT, I believe I previously messed up with the algebra. 1. The problem statement, all variables and given/known data Using the same meathod as in the text, calculate the entropy of mixing for a system of two monatomic ideal gases, A and B, whose relative proportion is arbitrary. Let N be the total number of molecules and let x be the fraction of these that are species B. You should find that ΔS=-Nk[xlnx+(1-x)ln(1-x)] Check that this expression reduces to the one given in the text when x= 1/2. 2. Relevant equations That S=Nk[ln(V(a/3n)^(3/2))+3/2] where a is just a whole bunch of stuff that I believe is irrelevant to the problem. PV=nkT might be useful also the fact that ln(x/y)=ln(x)-ln(y) 3. The attempt at a solution I know that the change of entropy will just be S_final-S_original but I do not know what really changes between the final and the original situations. Do I have to use the Ideal Gas law find out how big the volume would be?