Suppose that one mole of a monatomic perfect gas at 27°C and 1.00 atm pressure is expanded adiabatically (i.e. no heat transfer, so that the temperature must fall) in two different ways: (a) reversibly, to a final pressure of 0.5 atm, and (b) against a constant external pressure of 0.5 atm. (1) One point each. For the reversible case, determine the values of ΔU, ΔH, ΔS system, ΔS surroundings, and ΔS total. (2) Do the same for the irreversible expansion against a constant pressure of 0.5 atm. I think I'm fine with part A, but I am completely lost for part B and can't really start because I have no idea what formulas to use or where to even start. Unlike part A where ΔU=pdV=CdT and so allow you to integrate with respect to both sides, in part B the pressure is constant and so I'm not sure if I'm supposed to integrate the above formula. I first thought that the change in volume in the first part would be the same in the second part, which would allow me to easily find ΔU, but it seems this is not the case. I'm not looking for someone to do my work for me, but just lead me to some formulas concerning all the Δ's and show me where to start. Thanks.