Consider the following reactions.

$A{u}^{3+}+3e=Au,E^0=1\mathrm{.}5V(vs.$ NHE$)$

$A{u}^{3+}+3e=A{u}_{1}A{u}^{3+}+4C{l}^{-}=AuC{l}_4^{-},E^0=1\mathrm{.}0V(vs.$ NHE$)$

Calculate $K_f(15$pt$)$