A student balances $\text{MnO}_4^- + \text{SO}_3^{2-} \rightarrow \text{Mn}^{2+} + \text{SO}_4^{2-}$ in acid and writes $2\text{MnO}_4^- + 5\text{SO}_3^{2-} + 8\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{SO}_4^{2-} + 4\text{H}_2\text{O}$. What is wrong?