1. The problem statement, all variables and given/known data Two ideal gases are contained adiabatically and separated by an insulating, fixed piston that blocks the molecules of gas 2 but allows the molecules of gas 1 through(in both directions). The initial pressures, volumes, temperatures and number of molecules on each side is given. What is the equilibrium state? 2. Relevant equations I don't think they're explicitly necessary for the question. 3. The attempt at a solution The volumes remain the same, so we basically have 5 variables: The final pressures, temperatures and the net number of molecules that went through the piston. At equilibrium, the partial pressure of gas 1 must be equal on both sides, that gives 1 equation. The ideal gas law, applied to both sides gives 2 more, and the fact that the whole system's internal energy is constant gives 1 more. So I have 5 variables and only 4 equations. What am I missing?