How many moles of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$ can be reduced by 1 mole of ${\mathrm{Sn}}^{2+}$ ?

$$\begin{gathered} {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-} + 14{\mathrm{H}}^{+} + 6{\mathrm{e}}^{-} \stackrel{ }{\to } 2{\mathrm{Cr}}^{3+} + 7{\mathrm{H}}_2\mathrm{O} \\ \left({\mathrm{Sn}}^{2+} \stackrel{ }{\to }{\mathrm{Sn}}^{4+} + 2{\mathrm{e}}^{-}\right) \times 3 \\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_} \\ {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-} + 14{\mathrm{H}}^{+} + 3{\mathrm{Sn}}^{2+} \stackrel{ }{\to } 3{\mathrm{Sn}}^{4+}+2{\mathrm{Cr}}^{3+} + 7{\mathrm{H}}_2\mathrm{O} \\ \mathrm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_} \end{gathered}$$

It is clear from this equation that 3 moles of ${\mathrm{Sn}}^{2+}$ reduce one mole of ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-},$ hence 1 mol. of ${\mathrm{Sn}}^{2+}$ will reduce  $\frac{1}{3}$ moles of ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}.$