1. The problem statement, all variables and given/known data A monatomic ideal gas is compressed adiabatically. What is ΔS of the gas? n = 1.00 vi = 10.00 L vf = 4.00 L Constant External Pressure = 50.00 atm 2. Relevant equations ΔU = q + w = 0 + (-PextΔV) ΔS = ∫[(qrev)/T]dQ 3. The attempt at a solution In order to calculate ΔS you have connect two thermodynamic states with a reversible path. I assumed that the ΔS of the system is 0 because dQ is 0 for adiabatic processes). Then second state focuses on ΔS of the surroundings that has an external pressure associated with it. I found Tf = 322.06 K by using the ΔU equation I am not sure what to do at this point, because every single attempt I made has led me to a negative ΔS.