The first law of thermodynamics for isothermal process Q=−W
The W for isothermal reversible process is
W=−nRTlnV1V2
ΔS=TQ
ΔSsurrounding=−nRlnV1V2
=−1×8.314×2.303×log23
=−3.37JK−1
≃−3JK−1
The given process is mentioned as isothermal reversible process hence the ΔSsurrounding is calculated.