Preparation and standardization of potassium permanganate solution MCQs With Answer

Preparation and standardization of potassium permanganate solution MCQs With Answer

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Preparation and standardization of potassium permanganate solution MCQs With Answer

This concise guide focuses on the preparation and standardization of potassium permanganate solution, an essential topic in pharmaceutics and analytical chemistry for B. Pharm students. It covers practical steps for preparing KMnO4 solutions, stabilization, common impurities, storage, and detailed standardization techniques such as permanganometry using sodium oxalate or ferrous ammonium sulfate. Emphasis is placed on titration stoichiometry, acid medium choice, endpoint detection, calculations (molarity, normality, equivalent weight), and laboratory precautions to ensure accurate assay results. The content is designed to deepen practical understanding and exam readiness. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which compound is commonly used as a primary standard to standardize potassium permanganate solution?

  • Sodium oxalate (Na2C2O4)
  • Potassium dichromate (K2Cr2O7)
  • Potassium permanganate (KMnO4)
  • Sodium chloride (NaCl)

Correct Answer: Sodium oxalate (Na2C2O4)

Q2. Why is potassium permanganate not considered an ideal primary standard?

  • It is too inexpensive
  • It decomposes and often contains MnO2 impurities
  • It is colorless
  • It reacts only in alkaline medium

Correct Answer: It decomposes and often contains MnO2 impurities

Q3. In acidic permanganate titration with oxalate, which gas is evolved as the oxalate is oxidized?

  • Oxygen (O2)
  • Carbon dioxide (CO2)
  • Nitrogen (N2)
  • Chlorine (Cl2)

Correct Answer: Carbon dioxide (CO2)

Q4. What is the visible end point color in a permanganate titration in acidic medium?

  • Blue
  • Persistent light pink
  • Colorless
  • Green

Correct Answer: Persistent light pink

Q5. Which acid is recommended for permanganate titrations with oxalate and why?

  • Hydrochloric acid, because chloride stabilizes MnO4-
  • Sulfuric acid, because it provides an inert strong acid medium
  • Nitric acid, because it prevents gas evolution
  • Acetic acid, because it speeds the reaction

Correct Answer: Sulfuric acid, because it provides an inert strong acid medium

Q6. Why is the reaction between permanganate and oxalate often performed at 60–70°C?

  • To prevent color change
  • To decompose oxalate
  • To increase the reaction rate and ensure reproducible end point
  • To volatilize sulfuric acid

Correct Answer: To increase the reaction rate and ensure reproducible end point

Q7. How should a prepared KMnO4 solution be stored to maximize stability?

  • In a clear plastic bottle at room light
  • In a dark glass bottle, tightly closed, away from organic contaminants
  • Open to air on the bench
  • Heated to 50°C to prevent precipitation

Correct Answer: In a dark glass bottle, tightly closed, away from organic contaminants

Q8. Which unit accounts directly for the number of electrons transferred in a redox titration and is often used for KMnO4?

  • Molarity (M)
    • Percent concentration
  • Normality (N)
  • Molality (m)

Correct Answer: Normality (N)

Q9. A freshly prepared KMnO4 solution appears brownish instead of deep purple. What is the likely cause?

  • Excess potassium ions
  • Decomposition producing MnO2 or other reduced manganese species
  • Too much sulfuric acid present
  • Overheating the solution briefly

Correct Answer: Decomposition producing MnO2 or other reduced manganese species

Q10. Which ferrous salt is commonly used to standardize KMnO4 as an alternative primary standard?

  • Ferrous ammonium sulfate (FAS)
  • Ferrous sulfate heptahydrate (FeSO4·7H2O) without stabilizer
  • Ferric chloride (FeCl3)
  • Iron(III) oxide (Fe2O3)

Correct Answer: Ferrous ammonium sulfate (FAS)

Q11. In acidic medium, what is the stoichiometric molar ratio of Fe2+ to MnO4- in the redox reaction?

  • 1:1
  • 2:1
  • 5:1
  • 3:2

Correct Answer: 5:1

Q12. Which acid should be avoided when preparing or standardizing KMnO4 because it leads to interference by chloride oxidation?

  • Sulfuric acid (H2SO4)
  • Hydrochloric acid (HCl)
  • Perchloric acid (HClO4)
  • Phosphoric acid (H3PO4)

Correct Answer: Hydrochloric acid (HCl)

Q13. How does increasing temperature generally affect permanganate titration kinetics?

  • Slows the reaction and delays endpoint
  • Has no effect on reaction rate
  • Speeds up the reaction but may increase decomposition if excessive
  • Changes the stoichiometry

Correct Answer: Speeds up the reaction but may increase decomposition if excessive

Q14. Why is KMnO4 described as a self-indicating titrant?

  • It produces an external colorless complex at end point
  • It contains an added dye
  • Its own purple color disappears and a light pink persists at end point
  • It reacts to form a precipitate

Correct Answer: Its own purple color disappears and a light pink persists at end point

Q15. What is the equivalent weight of KMnO4 in acidic permanganate titrations (use molar mass ≈ 158.04 g/mol and n = 5)?

  • 158.04 g/eq
  • 31.61 g/eq
  • 79.02 g/eq
  • 15.804 g/eq

Correct Answer: 31.61 g/eq

Q16. After accurately weighing KMnO4 for solution preparation, what is the immediate next step in the laboratory procedure?

  • Dry the solid by heating strongly
  • Dissolve it in distilled water with minimal agitation
  • Add sulfuric acid directly to the solid
  • Place it in sunlight to purify

Correct Answer: Dissolve it in distilled water with minimal agitation

Q17. Which container material is most suitable for storing a standardized potassium permanganate solution?

  • Clear plastic bottle
  • Amber (dark) glass bottle
  • Aluminum flask
  • Open beaker covered with filter paper

Correct Answer: Amber (dark) glass bottle

Q18. Why is the KMnO4 solution often filtered after dissolution during preparation?

  • To remove dissolved KMnO4
  • To remove any insoluble manganese dioxide or particulate impurities
  • To sterilize the solution
  • To concentrate the solution

Correct Answer: To remove any insoluble manganese dioxide or particulate impurities

Q19. How many electrons does one MnO4- ion accept when reduced to Mn2+ in acidic solution?

  • 3 electrons
  • 1 electron
  • 5 electrons
  • 2 electrons

Correct Answer: 5 electrons

Q20. Which of the following is an advantage of using KMnO4 as a titrant in redox titrations?

  • Requires an external indicator
  • Is colorless throughout titration
  • Acts as a strong oxidizing agent and is self-indicating
  • Is inert and does not react with reducing agents

Correct Answer: Acts as a strong oxidizing agent and is self-indicating

Q21. Which is a major disadvantage of using potassium permanganate as a routine primary standard titrant?

  • It is too stable to standardize
  • It is not a primary standard because it decomposes and may contain impurities
  • It is colorless and hard to detect endpoint
  • It reacts only under alkaline conditions

Correct Answer: It is not a primary standard because it decomposes and may contain impurities

Q22. What is the recommended temperature range to perform a permanganate-oxalate titration for reliable kinetics?

  • 0–5°C
  • 20–25°C (room temperature)
  • 60–70°C
  • 90–100°C

Correct Answer: 60–70°C

Q23. Why is running a reagent blank important in permanganate standardization?

  • To increase KMnO4 concentration
  • To account for reducing impurities and correct titration values
  • To evaporate excess solvent
  • To change the stoichiometry of reaction

Correct Answer: To account for reducing impurities and correct titration values

Q24. How should the concentration of a stored KMnO4 solution be monitored over time?

  • Assume it remains constant for years
  • By periodic re-standardization against a primary standard
  • By measuring pH only
  • By observing color intensity visually without calibration

Correct Answer: By periodic re-standardization against a primary standard

Q25. What major error arises if hydrochloric acid is used instead of sulfuric acid during standardization?

  • Formation of insoluble permanganate salts
  • Oxidation of chloride to chlorine causing a false high consumption
  • Neutralization of KMnO4
  • No reaction occurs between KMnO4 and oxalate

Correct Answer: Oxidation of chloride to chlorine causing a false high consumption

Q26. If the molarity of a KMnO4 solution is 0.02 M, what is its normality for acidic titrations (n = 5)?

  • 0.004 N
  • 0.02 N
  • 0.10 N
  • 1.00 N

Correct Answer: 0.10 N

Q27. Which observation indicates that the titration of Fe2+ with KMnO4 has reached the end point?

  • The solution turns clear blue
  • A faint pink color remains for about 30 seconds
  • White precipitate forms
  • Solution becomes colorless

Correct Answer: A faint pink color remains for about 30 seconds

Q28. During standardization with sodium oxalate, what is essential to ensure complete reaction between oxalate and permanganate?

  • Maintain strongly alkaline conditions
  • Use chlorinated water
  • Heat the reaction mixture to about 60–70°C
  • Perform titration in complete darkness

Correct Answer: Heat the reaction mixture to about 60–70°C

Q29. When using KMnO4 as titrant, which external indicator is required?

  • Starch
  • Phenolphthalein
  • No external indicator is required
  • Methyl orange

Correct Answer: No external indicator is required

Q30. The analytical technique that describes KMnO4 titration for determining reducing substances is called:

  • Complexometry
  • Acid-base titrimetry
  • Permanganometry (redox titration)
  • Gravimetry

Correct Answer: Permanganometry (redox titration)

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