1. The problem statement, all variables and given/known data An evacuated insulated tank is filled with an ideal gas at T0, until inside pressure equals outside. No heat flow takes place. What is the temperature of the gas inside? Cv and Cp are known. 2. Relevant equations Um.... 3. The attempt at a solution P1/P2=(T2/T1)^(Cp/R), and P1=P2, so T1 equals T2? This just seems too simple, and wrong. The other approach I tried was comparing enthalpies, but I can't figure out how enthalpy changes. Is work, in fact, done by the gas filling the tank?