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## Homework Statement

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. (P

_{f}=2P

_{i}and V

_{f}=2V

_{i})

Find the net change in the entropy of the gas.

## Homework Equations

## The Attempt at a Solution

I found the entropy change for the first step. Let the volume between the two steps be V

_{m}.

TV

^{γ-1}= constant, so V

_{m}= 2

^{2.5}V

_{i}

ΔS = nR ln(V

_{m}/V

_{i}) = 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 ?