JEE Advanced 2016 · Paper 1, Chemistry question 34

Half-reactions.

• In neutral or faintly alkaline solution, $\ce{MnO4- + 2H2O + 3e- -> MnO2 + 4OH-}$.
• Thiosulphate is oxidised completely to sulphate. The average oxidation state of S goes from +2 to +6, which is 8 e⁻ per $\ce{S2O3^{2-}}$.

Electron balance. 8 mol of $\ce{MnO4-}$ accept 24 e⁻, which oxidise 3 mol of $\ce{S2O3^{2-}}$ to 6 mol of $\ce{SO4^{2-}}$:
$$\ce{8MnO4- + 3S2O3^{2-} + H2O -> 8MnO2 + 6SO4^{2-} + 2OH-}.$$

Answer: 6