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Question:

When XO2 is fused with an alkali metal hydroxide in presence of an oxidizing agent such as KNO3; a dark green product is formed which disproportionates in acidic solution to afford a dark purple solution. X is: Cr, V, Ti, Mn

Cr

Ti

V

Mn

Solution:

When fused with alkali metal hydroxide in presence of oxidising agent
MnO2 + 2KOH + KNO3 → K2MnO4 + KNO2 + H2O (green)
Disproportionation reaction is as following:
3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO2 + 2H2O (purple)
Hence, the correct option is Mn.