1. The problem statement, all variables and given/known data A piston‐cylinder initially contains 0.5 m3 of an ideal gas at 150 kPa and 20 degrees. The gas is suddenly subjected to a constant external pressure of 400 kPa and it is compressed in a manner that the final temperature is also 20oC. Assume the surroundings are also at 20 degrees. Take Cp=2.5 Ro. Determine: (i) What is the entropychange of (a) the system, (b) the surroundings and (c) the universe or total? (ii) Is the process reversible,irreversible or impossible? 2. Relevant equations U=Q-W; S=Q/T; 3. The attempt at a solution For the system, because the temperature didn't change after compression. Hence U=0; Q=W; W can be obtained by: W=PV = 400KPa(V2-V1）; V2 can be obtained by using ideal gas equation; PV= nRT; However, I encounter difficulties in doing so because, if so, there is only one Q value, Q leaking out from the system = Q entering the surrounding. Hence the entropy will be zero instead, which is not right by the answer given. The solution is given as follow and I dont really understand how they calculate the entropy change for system and equate it with isothermal process. And I doubt why the two Q values are different.