Calculate ∆G for the conversion of 1 mole of water at 100 °C and 1 atm to steam at 100 °C and 0.5 atm pressures.
∆G = (∆G for the conversion of H_2O (l) at 100 °C and 1 atm to steam at 100 °C and 1 atm).
+ (∆G for the transition of steam at 100 °C from 1 atm to 0.5 atm).
The first term on the RHS is zero, as H_2O (l) and H_2O (g) are in equilibrium at 100 °C and 1 atm pressure.
The second term is =2.303 \mathrm{~nRT} \log \frac{\mathrm{P}_{2}}{\mathrm{P}_{1}}
\begin{array}{l}\Delta G=2.303 \times 1 \times 1.987 \times 373 \times \log _{10} \frac{0.5}{1}\\ \\=2.303 \times 1 \times 1.987 \times 373 \times(-0.3010) \\ \\=-513.82 \mathrm{~cal}\end{array}