Question

The Number of moles Of ${\mathrm{KMnO}}_4$ required to oxidise one mole of $\mathrm{Fe}\left({\mathrm{C}}_2{\mathrm{O}}_4\right)$ is________.

• A. $0.2$
• B. $0.4$
• C. $0.6$
• D. $1.67$

Answer 1

The correct option is C

$0.6$

Explanation for Correct option:

(C) $0.6$

Step 1: Redox of $\mathrm{Fe}\left({\mathrm{C}}_2{\mathrm{O}}_4\right)$:

• The total number of electrons involved per mole oxidation of ${\mathrm{FeC}}_2{\mathrm{O}}_4\mathrm{to}{\mathrm{Fe}}^{3+}$and ${\mathrm{CO}}_2\mathrm{is}3$ moles.
• The oxidation of ferrous ions to ferric ions involves one mole of electrons.
  $$\begin{gathered} {\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)\to {\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right)+{\mathrm{e}}^{-} \\ \mathrm{Ferrous}\mathrm{Ferric} \end{gathered}$$
• The oxidation of oxalate ions to carbon dioxide involves two moles of electrons.
  $$\begin{gathered} {\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}\left(\mathrm{aq}\right)\to 2{\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{e}}^{-} \\ \mathrm{Oxalate}\mathrm{ion}\mathrm{carbon}\mathrm{dioxide} \end{gathered}$$
  Thus,
  $$\begin{gathered} {\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}\left(\mathrm{aq}\right)\to {\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)+3{\mathrm{e}}^{-}---\left(1\right) \\ \mathrm{Ferrous}\mathrm{ion}\mathrm{Oxalate}\mathrm{ion}\mathrm{Ferric}\mathrm{ion}\mathrm{carbon}\mathrm{dioxide} \end{gathered}$$

Step 2: Reduction of ${\mathrm{KMnO}}_4$:
In an acidic medium, one mole of ${\mathrm{KMnO}}_4$ requires $5$ moles of electrons.
$$\begin{gathered} {{\mathrm{MnO}}_4}^{-}\left(\mathrm{aq}\right)+8{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+5{\mathrm{e}}^{-}\to {\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)----\left(2\right) \\ (\mathrm{Permanganate}\mathrm{ion})(\mathrm{Manganese}\mathrm{ion}) \end{gathered}$$

Step 3: Writing overall equation:
To balance the number of electrons, multiply equation $\left(1\right)$ with $5$ and equation $\left(2\right)$ with $3$.
$$\begin{gathered} 5{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+5{\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}\left(\mathrm{aq}\right)\to 5{\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right)+10{\mathrm{CO}}_2\left(\mathrm{g}\right)+15{\mathrm{e}}^{-}----\left(3\right) \\ 3{{\mathrm{MnO}}_4}^{-}\left(\mathrm{aq}\right)+24{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+15{\mathrm{e}}^{-}\to 3{\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)+12{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)----\left(4\right) \end{gathered}$$
Add equations $\left(3\right)\&\left(4\right)$
$$\begin{gathered} 5{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+5{\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}\left(\mathrm{aq}\right)+3{{\mathrm{MnO}}_4}^{-}\left(\mathrm{aq}\right)+24{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to 5{\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right)+10{\mathrm{CO}}_2\left(\mathrm{g}\right)+3{\mathrm{Mn}}^{2+}\left(\mathrm{aq}\right)+12{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{Ferrous}\mathrm{ion})(\mathrm{Oxalate}\mathrm{ion})(\mathrm{Permanganate}\mathrm{ion})(\mathrm{Ferric}\mathrm{ion})(\mathrm{Carbon}\mathrm{dioxide})(\mathrm{Manganese}\mathrm{ion}) \end{gathered}$$

• Thus five moles of Ferrous oxalate are oxidized by three moles of ${\mathrm{KMnO}}_4$​ in acid medium.
• The number of moles of ${\mathrm{KMnO}}_4$​ required to oxidize one mole of ${\mathrm{Fe}}_2{\mathrm{Cr}}_2{\mathrm{O}}_4\mathrm{is}\frac{3}{5}=0.6$

Explanation for incorrect options:

Since, the required number of moles is $0.6$. Therefore, options (A), (B) and (D) are incorrect.

Hence, the correct option is (C) $0.6$