$C{r}_2{O}_7^{2-}+S{O}_3^{2-}\stackrel{H^{+}}{\to }C{r}^{3+}+S{O}_4^{2-}$.

The number of ${\text{H}}^{\text{+}}$ ions required to balance the above equation are

$$\begin{gathered} {\stackrel{+6}{Cr}}_2{O}_7^{2-}+\stackrel{+4}{S}{O}_3^{2-}\stackrel{H^{+}}{\to }C{r}^{3+}+\stackrel{+6}{S}{O}_4^{2-} \\ \mathrm{Change} \mathrm{in} \mathrm{oxidation} \mathrm{no}. \mathrm{of} \mathrm{Cr}=2\left(3\right)=6 \\ \mathrm{Change} \mathrm{in} \mathrm{oxidation} \mathrm{no}. \mathrm{of} \mathrm{S}=2 \\ \mathrm{Cross} \mathrm{multiply} \mathrm{the} \mathrm{oxidation} \mathrm{states}(\mathrm{in} \mathrm{the} \mathrm{ratio} 6:2=3:1) \\ {\stackrel{+6}{Cr}}_2{O}_7^{2-}+3\stackrel{+4}{S}{O}_3^{2-}\stackrel{H^{+}}{\to } 2C{r}^{3+}+\stackrel{+6}{3S}{O}_4^{2-} \end{gathered}$$
${\text{Cr}}_2{\text{O}}_7^{2-}+3{\text{SO}}_3^{2-}+8{\text{H}}^{+}\to 2{\text{Cr}}^{3+}+3{\text{SO}}_4^{2-}+4{\text{H}}_2\text{O}$