\(\mathrm{W}=\mathrm{n} \mathrm{RT} \log _{\mathrm{e}} \frac{\mathrm{V}_{2}}{\mathrm{V}_{1}}\)
For, \(n=1\)
\(\mathrm{W}=\mathrm{RT} \ln \frac{\mathrm{V}_{2}}{\mathrm{V}_{1}}\)
For an isothermal process, \(\Delta \mathrm{U}=0\) \(\Delta \mathrm{Q}=\Delta \mathrm{U}+\mathrm{W}=0+\mathrm{RT} \ln \frac{\mathrm{V}_{2}}{\mathrm{V}_{1}}\)
\(\therefore\) Entropy \(=\frac{\Delta Q}{T}=R \ln \frac{V_{2}}{V_{1}}\)