Entropy change in isothermal process

2016 Phase II 1 question

1. 1 Thermodynamics
   For a sample of perfect gas when its pressure is changed 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 $\Delta S = RT\ln\left(\frac{p_i}{p_f}\right)$ C $\Delta S = nR\ln\left(\frac{p_f}{p_i}\right)$ D $\Delta S = nR\ln\left(\frac{p_i}{p_f}\right)$
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