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Preparation, properties and uses of potassium dichromate and permanganate: NEET MCQs with answers

90 questions on this topic of d- and f-Block Elements (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following organic conversions is typically achieved using potassium permanganate?

  1. Dehydration of primary alcohols to alkenes
  2. Reduction of nitrobenzene to aniline
  3. Cleavage and oxidation of an aromatic side chain (alkyl group) to a carboxyl group (-COOH)
  4. Conversion of carboxylic acids into acid chlorides
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Answer: (C) Cleavage and oxidation of an aromatic side chain (alkyl group) to a carboxyl group (-COOH)

Alkaline or acidic KMnO₄ is a powerful oxidizing agent used in organic chemistry to vigorously oxidize alkyl side chains on aromatic rings (regardless of chain length, provided a benzylic hydrogen is present) to benzoic acid (-COOH). It does not act as a reducing or dehydrating agent.

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2. Given the following statements about potassium dichromate and potassium permanganate: Statement I: Potassium permanganate solutions are standardized against sodium oxalate in the presence of dilute sulfuric acid. Statement II: Nitric acid can be used in place of sulfuric acid to acidify KMnO₄ during titrations. Choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because sodium oxalate is a primary standard used to standardize KMnO₄ solutions. Statement II is incorrect because HNO₃ is itself a strong oxidizing agent and interferes in the redox titration by reacting with the reducing agent.

hard

3. In the dichromate ion, the two tetrahedra share:

  1. An edge
  2. A face
  3. No atoms
  4. One corner, with a Cr-O-Cr bond angle of 126°
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Answer: (D) One corner, with a Cr-O-Cr bond angle of 126°

Cr₂O₇²⁻ consists of two CrO₄ tetrahedra sharing one corner oxygen, with a Cr-O-Cr bond angle of 126°. Chromate is a single tetrahedron.

NCERT Class 12

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4. In alkaline solution, potassium permanganate oxidizes thiosulfate ions (S₂O₃²⁻). What is the oxidation state of sulfur in the major oxidation product formed?

  1. +6
  2. +4
  3. +2.5
  4. 0
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Answer: (A) +6

In neutral or faintly alkaline solution, permanganate oxidizes thiosulfate quantitatively to sulfate: 8MnO₄⁻ + 3S₂O₃²⁻ + H₂O → 8MnO₂ + 6SO₄²⁻ + 2OH⁻. In the sulfate ion (SO₄²⁻), the oxidation state of sulfur is +6.

easy

5. When solid potassium permanganate (KMnO₄) is heated strongly to 513 K, it decomposes to give:

  1. K₂O, MnO₂, and O₂
  2. K₂O, Mn₂O₃, and O₂
  3. K₂MnO₄, MnO, and O₃
  4. K₂MnO₄, MnO₂, and O₂
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Answer: (D) K₂MnO₄, MnO₂, and O₂

Potassium permanganate decomposes upon heating at 513 K according to the reaction: 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂. This reaction yields dark green potassium manganate, manganese dioxide, and dioxygen gas.

hard

6. What volume (in mL) of 0.02 M KMnO₄ solution in acidic medium is required to oxidize completely 50 mL of 0.05 M potassium nitrite (KNO₂) solution?

  1. 50 mL
  2. 25 mL
  3. 75 mL
  4. 100 mL
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Answer: (A) 50 mL

The redox reaction in acidic medium is: 2MnO₄⁻ + 5NO₂⁻ + 6H⁺ → 2Mn²⁺ + 5NO₃⁻ + 3H₂O. Millimoles of NO₂⁻ = 50 mL × 0.05 M = 2.5 mmol. From stoichiometry, millimoles of MnO₄⁻ required = (2/5) × 2.5 = 1.0 mmol. Volume of 0.02 M KMnO₄ = 1.0 mmol / 0.02 M = 50 mL.

easy

7. Which of the following compounds is formed when cold, concentrated H₂SO₄ is added to potassium permanganate crystals?

  1. Manganese dioxide (MnO₂)
  2. Manganese heptoxide (Mn₂O₇)
  3. Manganese sulfate (MnSO₄)
  4. Manganese trioxide (MnO₃)
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Answer: (B) Manganese heptoxide (Mn₂O₇)

When KMnO₄ is treated with cold concentrated H₂SO₄, the green oily compound manganese heptoxide (Mn₂O₇) is produced according to 2KMnO₄ + 2H₂SO₄ → Mn₂O₇ + 2KHSO₄ + H₂O. Mn₂O₇ is a highly explosive covalent oxide.

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8. In acidic medium, potassium dichromate reacts with nitrite ions (NO₂⁻) to produce nitrate ions (NO₃⁻) and Cr³⁺ ions. How many moles of H⁺ ions are consumed in the balanced ionic equation per mole of Cr₂O₇²⁻ reduced?

  1. 8
  2. 14
  3. 6
  4. 10
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Answer: (A) 8

The half-reactions are: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, and 3(NO₂⁻ + H₂O → NO₃⁻ + 2H⁺ + 2e⁻). Combining them gives: Cr₂O₇²⁻ + 3NO₂⁻ + 8H⁺ → 2Cr³⁺ + 3NO₃⁻ + 4H₂O. Therefore, exactly 8 moles of H⁺ are consumed per mole of Cr₂O₇²⁻.

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9. In alkaline or neutral medium, potassium permanganate oxidizes iodide (I⁻) quantitatively. What is the mole ratio of KMnO₄ to I⁻ consumed in this balanced redox reaction?

  1. 5 : 1
  2. 1 : 1
  3. 2 : 1
  4. 2 : 3
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Answer: (C) 2 : 1

In neutral or faintly alkaline medium, permanganate oxidizes I⁻ to iodate (IO₃⁻): 2MnO₄⁻ + H₂O + I⁻ → 2MnO₂ + 2OH⁻ + IO₃⁻. The stoichiometric ratio of MnO₄⁻ to I⁻ is 2 : 1.

easy

10. When solid potassium permanganate is heated strongly to about 513 K, which product mixture is formed?

  1. MnO₂, K₂O, and O₂
  2. K₂O, Mn₂O₃, and O₂
  3. K₂MnO₄, MnO₂, and O₂
  4. K₂MnO₄ and Mn₂O₇
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Answer: (C) K₂MnO₄, MnO₂, and O₂

On heating at 513 K, potassium permanganate decomposes to give potassium manganate, manganese dioxide, and oxygen gas: 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂.

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11. In the industrial preparation of potassium dichromate, finely powdered chromite ore is fused with sodium carbonate in the presence of excess air. What is the stoichiometric coefficient of O₂ in the balanced chemical equation for this fusion per mole of FeCr₂O₄?

  1. 1.75
  2. 2.00
  3. 3.50
  4. 4.00
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Answer: (A) 1.75

The balanced reaction for the fusion of chromite ore is: 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂. Dividing throughout by 4 gives 7/4 = 1.75 moles of O₂ per mole of FeCr₂O₄.

easy

12. During the commercial preparation of potassium permanganate, pyrolusite ore (MnO₂) is fused with KOH in the presence of air to give a green mass of K₂MnO₄. On an industrial scale, the oxidation of manganate(VI) to permanganate(VII) is subsequently carried out by:

  1. Electrolytic oxidation in alkaline solution
  2. Treating the solution with concentrated hydrochloric acid
  3. Passing sulfur dioxide gas into the alkaline solution
  4. Disproportionation by adding excess potassium hydroxide
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Answer: (A) Electrolytic oxidation in alkaline solution

Commercially, potassium permanganate is prepared by the alkaline oxidative fusion of MnO₂ followed by the electrolytic oxidation of manganate(VI) to permanganate(VII) in alkaline solution. In the laboratory, chemical oxidation or disproportionation in acidic/neutral media can be used, but large-scale production relies on electrolytic oxidation.

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