Hess law states that,

For the reaction,

$\frac{1}{2}{\mathrm{H}}_2\left(\mathrm{g}\right)+\frac{1}{2}{\mathrm{Cl}}_2\left(\mathrm{g}\right)⟶{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$

${\mathrm{\Delta G}}_{\text{recation }}°=-131.23 \mathrm{kJ} {\mathrm{mol}}^{-1}$

The value of ${\mathrm{\Delta G}}_{\text{formation }}°$ of ${\mathrm{Ag}}^{+}$ (aq) shall be given by, (if ${\mathrm{\Delta G}}_{\mathrm{f}}°\left({\mathrm{H}}^{+}\mathrm{aq}\right)=0$)