For the given reaction:

$\begin{array}{r}{\mathrm{H}}_2\left(g\right)+{\mathrm{Cl}}_2\left(g\right)⟶2{\mathrm{H}}^{+}\left(aq\right)+2{\mathrm{Cl}}^{-}\left(aq\right)\\ \mathrm{\Delta }{G}^{\circ }=-262.4\mathrm{kJ}\end{array}$

The value of Gibbs free energy of formation $\left(\mathrm{\Delta }{G}_f^{\circ }\right)$ for the ion ${\mathrm{Cl}}^{-}\left(aq\right)$ is :