The change in the entropy of a $1$ mole of an ideal gas which went through an isothermal process from an initial state $(P_1, V_1,T)$ to the final state $(P_2, V_2,T)$ is equal to
AIIMS 2010, Medium
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Workdone by $n$ moles of a gas when its volume changes from $\mathrm{V}_{1}$ to $\mathrm{V}_{2}$ is,

$\mathrm{W}=\mathrm{n} \mathrm{RT} \log _{\mathrm{e}} \frac{\mathrm{V}_{2}}{\mathrm{V}_{1}}$

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 $=\frac{\Delta Q}{T}=R \ln \frac{V_{2}}{V_{1}}$

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