An ideal gas of mass $m$ and temperature $T_1$ undergoes a reversible isothermal process from an initial pressure $P_1$ to final pressure $P_2$. The hear loss during the process is $Q$. The entropy change $\Delta S$ of the gas is
- $mR \: \ln \bigg( \dfrac{P_2}{P_1} \bigg) \\$
- $mR \: \ln \bigg( \dfrac{P_1}{P_2} \bigg) \\$
- $mR \: \ln \bigg( \dfrac{P_2}{P_1} \bigg) - \dfrac{Q}{T_1} \\$
- $\text{zero}$