1. The problem statement, all variables and given/known data Temperature, pressure and volume measurements performed on 1 kg of a simple compressible substance in three stable equilibrium states yield the following results. State 1 (T1=400 C , V1= 0,10 m3, P1=3 MPa) State 2 (T1=400 C , V1= 0,08 m3, P1=3,5 MPa) State 3 (T1=500 C , V1= 0,10 m3, P1=3,5 MPa) Estimate the difference in entropy S2-S1 2. Relevant equations We don't know the gas. So I can't assume this is an ideal gas and I can't go to thermodynamics tables. I don't know the relevant equation. 3. The attempt at a solution First, I didn't get why the question identifies state 3. I think we can completely ignore state 3 because the question is entropy difference between state 2 and state 1. This is clearly a compression process, (volume decreases, pressure increases) but temperature stays still. But when a gas is compressed, its pressure and temperature rises. So there must be heat transfer going on. Entropy change = Sgen + Q/T But we don't know the entropy generation so we can't go from here. I assume I need a relation related with volume and pressure that yields entropy but in constant temperature. Is there a relation like that?