For a sample of a perfect gas whose pressure changes isothermally from $P_i$ to $P_f$, the entropy change is given by:
- A$\Delta S=nRT\ln\!\left(\frac{P_f}{P_i}\right)$
- B$\Delta S=nRT\ln\!\left(\frac{P_i}{P_f}\right)$
- C$\Delta S=nR\ln\!\left(\frac{P_f}{P_i}\right)$
- D$\Delta S=nR\ln\!\left(\frac{P_i}{P_f}\right)$
✓ Correct answer: D
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