1. The problem statement, all variables and given/known data A piston cylinder device contains 1.2kg of nitrogen at 120kPa and 300K. Gas is compressed slowly in a polytropic process during which PV^1.3 = constant. The proces ends when the volume is reduced by one half. What is the entropy change? 2. Relevant equations (P2/P1) = (V1/V2)^n 3. The attempt at a solution Assuming V1 = 1 and therefore V2 = 0.5, I have worked out P2 as 295.5kPa but I am stuck on what to do next.