Question

 Write the chemical equation for the preparation of 4 KMnO from 2. MnO Why does purple colour of acidified permanganate solution decolourise when it oxidises Fe^2- to Fe³⁺ ? 

Answer 1

Chemical equations for the preparation of KMnO₄ from MnO₂ 
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
${\displaystyle 3Mn{O}_4^2+4H\to 2Mn{O}_4^{- }+2Mn{O}_4^{-}+Mn{O}_2+2H_{2}O }$

$$\mathrm{MnO}{\phantom{A}}_2\underset{\mathrm{with}\mathrm{air}\mathrm{or}\mathrm{KNO}{\phantom{A}}_3}{\overset{\mathrm{Fused}\mathrm{with}\mathrm{oxidised}}{\to }}Mn{O}_4^{2-}$$ maganate ion 

$$\underset{\mathrm{Manganate}}{\mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{2-}}\underset{\mathrm{in}\mathrm{alkaline}\mathrm{solution}}{\overset{\mathrm{Electrolytic}\mathrm{oxidation}}{\to }}\mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{-}$$ permanganateion

Purple colour of acidified permanganate solution decolourise when it oxidises Fe2- + Fe³⁺ 
In acidic medium, Reduction

KMnO₄ is behaving as oxidising Agent.
So, purple colour of acidified 4 KMnO solutions decolourises.