${}_{r}G,E_{\text{c}\text{e}\text{l}\text{l}},$ and $K$

$25.$ Determine the values of ${}_{r}G$ for the following reactions carried out in voltaic cells.

$($a$)2Al($s$)+3C{u}^{2+}(aq)longrightarrow2A{l}^{3+}(aq)+3Cu(s)$

$($b$)O_2($g$)+4I^{-}($aq$)+4H^{+}($aq$)longrightarro{w}_{2H_{2}O(l)+2I_2((s))}$

$($c$)C{r}_2{O}_7^{2-}($aq$)+14H^{+}($aq$)+6Ag(s)longrightarrow$

$2C{r}^{3+}(aq)+6A{g}^{+}(aq)+7H_{2}O(l)$