Balance the following redox reaction in acidic medium and determine the stoichiometric coefficient of $\mathrm{H}_2 \mathrm{O}$ in the final balanced equation.

$$ \mathrm{MnO}_4^{-}(a q)+\mathrm{Fe}^{2+}(a q) \longrightarrow \mathrm{Mn}^{2+}(a q)+\mathrm{Fe}^{3+}(a q) $$

Calculate the volume of $M / 10 \mathrm{KMnO}_4$ needed to react with 20 mL of $M / 2 \mathrm{FeSO}_4$ in acidic medium.

What is the stoichiometric coefficient of $ \mathrm{SO}_{2} $ in the following balance reaction?

$ \mathrm{MnO}_{4}^{-}(a q)+\mathrm{SO}_{2}(\mathrm{~g}) \longrightarrow \mathrm{Mn}^{2+}(a q)^{-}+\mathrm{HSO}_{4}^{-}(a q) $

(in acidic solution)