Entropy changes for ideal gas are as follows:

(i) Entropy change is terms of temperature and volume is given by $\mathrm{\Delta }S=n{C}_v\ln \frac{T_2}{T_1}+nR\ln \frac{V_2}{V_1}$

It is for n moles of an ideal gas from V₁ to V₂ when temperature changes from T₁; to T₂. C_v is the heat capacity at constant volume.

(ii) Entropy change in terms of temperature and pressure $\mathrm{\Delta S}={\mathrm{nC}}_{\mathrm{p}}\ln \frac{{\mathrm{T}}_2}{{\mathrm{T}}_1}+\mathrm{nRln}\frac{{\mathrm{P}}_2}{{\mathrm{P}}_1} (\mathrm{For} \mathrm{n} \mathrm{moles} \mathrm{of} \mathrm{gas})$

C_p is the molar heat capacity at constant pressure.