Required equation is given below,
$Zn +2 OH ^{-} \longrightarrow ZnO _{2}^{2-}+2 H ^{+}+2 e^{-}$
$NO _{3}^{-}+8 H ^{+}+8 e^{-} \longrightarrow OH ^{-}+2 H _{2} O + NH _{3}$
From the above equation
$\because 8$ moles of electron absorbed by $85\, g$ of $NaNO _{3}$
$\therefore 1$ mole of electron absorbed by $\frac{85}{8} g$ of $NaNO _{3}$
$=10.625\, g$