Acidified KMnO4 and neutral KMnO4 are…

Acidified KMnO4 and neutral KMnO4 are used as oxidants. Select the correct statement(s) about the stoichiometry of reactions of the permanganate ion…

One or more options correct

Acidified

\mathrm{KMnO_4}

and neutral

\mathrm{KMnO_4}

are used as oxidants. Select the correct statement(s) about the stoichiometry of reactions of the permanganate ion,

\mathrm{MnO_4^-}

A. In acidic solution, $1$ mol of $\mathrm{MnO_4^-}$ oxidises $5$ mol of $\mathrm{Fe^{2+}}$ to $\mathrm{Fe^{3+}}$ .
B. In acidic solution, $1$ mol of $\mathrm{MnO_4^-}$ oxidises $5$ mol of $\mathrm{C_2O_4^{2-}}$ to $\mathrm{CO_2}$ .
C. In neutral solution, $1$ mol of $\mathrm{MnO_4^-}$ (reduced to $\mathrm{MnO_2}$ ) oxidises $1.5$ mol of $\mathrm{H_2O_2}$ to $\mathrm{O_2}$ .
D. In strongly alkaline solution, $1$ mol of $\mathrm{MnO_4^-}$ (reduced to $\mathrm{MnO_4^{2-}}$ ) oxidises $0.5$ mol of $\mathrm{I^-}$ to $\mathrm{I_2}$ .

Show answer and solution

Answer: A, C

Count electrons. In acid,

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

, so each permanganate accepts

5e^-

. (A)

\mathrm{Fe^{2+}}\to\mathrm{Fe^{3+}}

gives

1e^-

each, so

5

mol are oxidised: correct. (B)

\mathrm{C_2O_4^{2-}}\to2\mathrm{CO_2}

gives

2e^-

per ion, so

1

mol of permanganate oxidises

5/2=2.5

mol of oxalate, not

5

mol: incorrect. (C) In neutral medium

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

takes

3e^-

, while

\mathrm{H_2O_2}\to\mathrm{O_2}

gives

2e^-

, so

3/2=1.5

mol of peroxide: correct. (D) In strongly alkaline medium

\mathrm{MnO_4^-}+e^-\to\mathrm{MnO_4^{2-}}

takes only

1e^-

, and

\mathrm{I^-}\to\tfrac12\mathrm{I_2}

gives

1e^-

per ion, so

1

mol of permanganate oxidises

1

mol of iodide, not

0.5

mol: incorrect. Correct options: A and C.

Original question written in the JEE Advanced 2016 Paper 2 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake