One mole of a monatomic gas first expands isothermally, then contracts adiabatically. The final pressure and volume of the gas are twice its initial pressure and volume. (Pf=2Pi and Vf=2Vi)
Find the net change in the entropy of the gas.
The Attempt at a Solution
I found the entropy change for the first step. Let the volume between the two steps be Vm.
TVγ-1 = constant, so Vm = 22.5 Vi
ΔS = nR ln(Vm/Vi) = 1.73 nR
But I have no idea about the entropy change of the second step. What is the entropy change for an adiabatic process? Isn't dQ=0 ?