Correct statement(s) about the… | JEE Advanced 2017 P2 Q14

Correct statement(s) about the chemistry of manganese and chromium oxoanions is(are)

One or more options correct

Correct statement(s) about the chemistry of manganese and chromium oxoanions is(are)

A. In acidic medium, one mole of $\mathrm{MnO_4^-}$ oxidises five moles of $\mathrm{Fe^{2+}}$
B. In neutral medium, $\mathrm{MnO_4^-}$ is reduced to $\mathrm{MnO_2}$ , accepting three electrons per ion
C. In acidic solution, three moles of $\mathrm{MnO_4^{2-}}$ give two moles of $\mathrm{MnO_4^-}$ and one mole of $\mathrm{MnO_2}$
D. Conversion of orange $\mathrm{Cr_2O_7^{2-}}$ to yellow $\mathrm{CrO_4^{2-}}$ on adding alkali is a redox reaction

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Answer: A, B, C

Option A is correct:

\mathrm{MnO_4^-+8H^++5e^-\to Mn^{2+}+4H_2O}

, so each permanganate accepts 5 electrons and oxidises 5

\mathrm{Fe^{2+}}

(each losing one electron). Option B is correct:

\mathrm{MnO_4^-+2H_2O+3e^-\to MnO_2+4OH^-}

in neutral or weakly alkaline solution, so 3 electrons are accepted. Option C is correct: manganate is stable only in strongly alkaline solution and disproportionates in acid,

\mathrm{3MnO_4^{2-}+4H^+\to2MnO_4^-+MnO_2+2H_2O}

(Mn +6 goes to +7 and +4; the charge is balanced as

-6+4=-2

on both sides). Option D is wrong:

\mathrm{Cr_2O_7^{2-}+2OH^-\rightleftharpoons2CrO_4^{2-}+H_2O}

keeps chromium at +6, so it is an acid-base (pH-dependent) equilibrium, not a redox process.

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