The reduction potential (E⁰, in V) of  ${\mathrm{MnO}}_4^{-}$−(aq)/Mn(s) is ______.
[Given:${\mathrm{E}}_{\left({\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right)/{\mathrm{MnO}}_2\left(\mathrm{s}\right)\right)}^0=1.68\mathrm{V};{\mathrm{E}}_{\left({\mathrm{MnO}}_2\left(\mathrm{s}\right)/{\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)\right)}^0=1.21\mathrm{V};{\mathrm{E}}_{\left({\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)/\mathrm{Mn}\left(\mathrm{s}\right)\right)}^0=-1.03\mathrm{V}$]

$\begin{array}{r}\left(1\right){\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right)+4{\mathrm{H}}^{+}+3\mathrm{e}⟶{\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O};{\mathrm{E}}^{\circ }=1.68\mathrm{V}\\ {\mathrm{\Delta G}}_1^0=-3\mathrm{F}\left(1.68\right)=-5.04\mathrm{F}\end{array}$
$\begin{array}{r}\left(2\right){\mathrm{MnO}}_2\left(\mathrm{s}\right)+4{\mathrm{H}}^{+}+2\mathrm{e}⟶{\mathrm{Mn}}^{2+}+2{\mathrm{H}}_2\mathrm{O};{\mathrm{E}}^{\circ }=1.21\mathrm{V}\\ {\mathrm{\Delta G}}_2^0=-2\mathrm{F}\left(1.21\right)=-2.42\mathrm{F}\end{array}$
$\begin{array}{l}\left(3\right){\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)+2\mathrm{e}⟶\mathrm{Mn}\left(\mathrm{s}\right);\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{E}}^{\circ }=-1.03\mathrm{V}\\ {\mathrm{\Delta G}}_3^0=-2\mathrm{F}(-1.03)=+2.06\mathrm{F}\end{array}$
Adding (1), (2) and (3),
$\begin{array}{l}{\mathrm{MnO}}_4^{-}\left(\mathrm{aq}\right)+8{\mathrm{H}}^{+}+7\mathrm{e}⟶\mathrm{Mn}\left(\mathrm{s}\right)+4{\mathrm{H}}_2\mathrm{O}\\ \begin{array}{rl}\mathrm{\Delta G}& ={\mathrm{\Delta G}}_1^0+{\mathrm{\Delta G}}_2^0+{\mathrm{\Delta G}}_3^0\\ & =(-5.04-2.42+2.06)\mathrm{F}\\ -7{\mathrm{FE}}^{\circ }=-5.4\mathrm{F}& \end{array}\\ {\mathrm{E}}^{\circ }=0.77\mathrm{V}\end{array}$