Chapter 17.3, Problem 17.5P

The standard cell potential at 25 °C is 1.21 V for the reaction

     ${\text{Cr}}_{\text{2}}{\text{O}}_{\text{7}}{}^{2-}\left(aq\right)+3{\text{Sn}}^{\text{2+}}\left(aq\right)+14{\text{H}}^{\text{+}}\left(aq\right)\to 2{\text{Cr}}^{\text{3+}}\left(aq\right)+3{\text{Sn}}^{\text{4+}}\left(aq\right)+7{\text{H}}_{\text{2}}\text{O}\left(l\right)$

What is the standard free-energy change for this reaction at 25 °C?