For the following equilibrium at 373 K
${\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\rightleftharpoons {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}{\mathrm{\Delta G}}^{\mathrm{o}}=?$

373K is boiling point of water 
At 1 atm V,P.   
$\begin{array}{l}{\mathrm{K}}_{\mathrm{p}}=\frac{{\mathrm{P}}_{{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}}}{{\mathrm{P}}_{{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}}}=1.0\\ {\mathrm{\Delta G}}^{\mathrm{o}}=-2.303\mathrm{RTlog}\mathrm{Kp}\end{array}$
=-2.303RT log1
=0