For a sample of perfect gas when its pressure is changed isothermally from $p_i\text{ to }{p}_f$, the entropy change is given by

• A

  $\mathrm{\Delta S}=\mathrm{nRln}\left(\frac{{\mathrm{p}}_{\mathrm{f}}}{{\mathrm{p}}_{\mathrm{i}}}\right)$

• B

  $\mathrm{\Delta S}=\mathrm{nRln}\left(\frac{{\mathrm{p}}_{\mathrm{i}}}{{\mathrm{p}}_{\mathrm{f}}}\right)$

• C

  $\mathrm{\Delta S}=\mathrm{nRTln}\left(\frac{{\mathrm{p}}_{\mathrm{f}}}{{\mathrm{p}}_{\mathrm{i}}}\right)$

• D

  $\mathrm{\Delta S}=\mathrm{RTln}\left(\frac{{\mathrm{p}}_{\mathrm{i}}}{{\mathrm{p}}_{\mathrm{f}}}\right)$