Question:
When \(MnO_2\) is heated with \(PbO_2\) and conc. \(HNO_3\), pink colour is obtained due to formation of:
When \(MnO_2\) is heated with \(PbO_2\) and conc. \(HNO_3\), pink colour is obtained due to formation of:
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Pink/purple colour in acidic medium \(\rightarrow\) \(HMnO_4\) (permanganic acid).
\(PbO_2\) + conc. \(HNO_3\) is a powerful oxidizing combination.
Updated On: Apr 16, 2026
- \(KMnO_4\)
- \(HMnO_4\)
- \(Pb(MnO_4)_2\)
- \(PbMnO_4\)
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The Correct Option is B
Solution and Explanation
Concept:
In this reaction, manganese dioxide (\(MnO_2\)) is oxidized from the +4 oxidation state to the +7 oxidation state. Concentrated \(HNO_3\) and \(PbO_2\) (lead dioxide) act as strong oxidizing agents in an acidic medium.
Step 1: Identify the oxidizing conditions. \(PbO_2\) is a very strong oxidizing agent, especially in the presence of concentrated \(HNO_3\). It can oxidize \(Mn^{4+}\) (in \(MnO_2\)) to \(Mn^{7+}\).
Step 2: Write the reaction. The oxidation of \(MnO_2\) in acidic medium produces permanganic acid (\(HMnO_4\)): \[ 2MnO_2 + 3PbO_2 + 6HNO_3 \rightarrow 2HMnO_4 + 3Pb(NO_3)_2 + 2H_2O \] Alternatively: \[ MnO_2 + PbO_2 + 2HNO_3 \rightarrow HMnO_4 + Pb(NO_3)_2 + H_2O \]
Step 3: Identify the pink colour. Permanganic acid (\(HMnO_4\)) is a strong acid that dissociates to give \(MnO_4^-\) (permanganate ions) in solution. Permanganate ions are known for their characteristic pink to purple colour. This colour is observed in the solution.
Step 4: Why not other options?
• (A) \(KMnO_4\) — Potassium permanganate is a salt that also gives a purple colour, but potassium ions (\(K^+\)) are not present in the reaction mixture (no potassium source).
• (B) \(HMnO_4\) — Correct; formed in the acidic medium (conc. \(HNO_3\)).
• (C) \(Pb(MnO_4)_2\) — Lead permanganate is a solid that is also purple, but it is not the primary product formed under these conditions.
• (D) \(PbMnO_4\) — This compound does not contain manganese in the +7 oxidation state.
Step 1: Identify the oxidizing conditions. \(PbO_2\) is a very strong oxidizing agent, especially in the presence of concentrated \(HNO_3\). It can oxidize \(Mn^{4+}\) (in \(MnO_2\)) to \(Mn^{7+}\).
Step 2: Write the reaction. The oxidation of \(MnO_2\) in acidic medium produces permanganic acid (\(HMnO_4\)): \[ 2MnO_2 + 3PbO_2 + 6HNO_3 \rightarrow 2HMnO_4 + 3Pb(NO_3)_2 + 2H_2O \] Alternatively: \[ MnO_2 + PbO_2 + 2HNO_3 \rightarrow HMnO_4 + Pb(NO_3)_2 + H_2O \]
Step 3: Identify the pink colour. Permanganic acid (\(HMnO_4\)) is a strong acid that dissociates to give \(MnO_4^-\) (permanganate ions) in solution. Permanganate ions are known for their characteristic pink to purple colour. This colour is observed in the solution.
Step 4: Why not other options?
• (A) \(KMnO_4\) — Potassium permanganate is a salt that also gives a purple colour, but potassium ions (\(K^+\)) are not present in the reaction mixture (no potassium source).
• (B) \(HMnO_4\) — Correct; formed in the acidic medium (conc. \(HNO_3\)).
• (C) \(Pb(MnO_4)_2\) — Lead permanganate is a solid that is also purple, but it is not the primary product formed under these conditions.
• (D) \(PbMnO_4\) — This compound does not contain manganese in the +7 oxidation state.
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