Preparation and standardization of oxalic acid solution MCQs With Answer

Preparation and standardization of oxalic acid solution MCQs With Answer

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Preparation and standardization of oxalic acid solution MCQs With Answer — This focused introduction explains practical and theoretical aspects of preparing and standardizing oxalic acid solutions for volumetric redox titrations, especially with potassium permanganate. B. Pharm students will learn about selection of primary standards, accurate weighing of oxalic acid dihydrate, stoichiometry of the oxalate–permanganate reaction, effect of temperature and acid medium, endpoint recognition, calculations of molarity and normality, common errors, and safety/handling. Keywords: oxalic acid dihydrate, primary standard, standardization, titration, potassium permanganate, molarity, normality, stoichiometry, endpoint, B. Pharm. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Why is oxalic acid dihydrate commonly used as a primary standard for standardizing KMnO4?

  • It can be obtained in high purity with known composition and gives reproducible titrations
  • It is a strong oxidizing agent that reacts with KMnO4
  • It is highly volatile and evaporates readily to constant weight
  • It changes color at the endpoint, eliminating the need for titrant

Correct Answer: It can be obtained in high purity with known composition and gives reproducible titrations

Q2. What is the chemical formula of oxalic acid dihydrate used in standardization?

  • H2C2O4
  • H2C2O4·2H2O
  • CO2H2·2H2O
  • C2O4H4

Correct Answer: H2C2O4·2H2O

Q3. Approximate molar mass (g mol−1) of oxalic acid dihydrate (H2C2O4·2H2O)?

  • 90.03 g mol−1
  • 126.07 g mol−1
  • 150.12 g mol−1
  • 100.05 g mol−1

Correct Answer: 126.07 g mol−1

Q4. In the redox reaction between oxalic acid and permanganate in acidic medium, what is the stoichiometric mole ratio of H2C2O4 : KMnO4?

  • 1 : 1
  • 5 : 2
  • 2 : 5
  • 3 : 1

Correct Answer: 5 : 2

Q5. Why is the titration of oxalic acid with potassium permanganate performed at elevated temperature (around 60 °C)?

  • To prevent decomposition of oxalic acid
  • To accelerate the slow redox reaction and obtain a sharp endpoint
  • To evaporate excess solvent for concentration
  • To change the color of permanganate for visibility

Correct Answer: To accelerate the slow redox reaction and obtain a sharp endpoint

Q6. Which acid is preferred to acidify the reaction mixture when standardizing KMnO4 with oxalic acid?

  • Hydrochloric acid (HCl)
  • Nitric acid (HNO3)
  • Sulfuric acid (H2SO4)
  • Acetic acid (CH3COOH)

Correct Answer: Sulfuric acid (H2SO4)

Q7. Is an external indicator required when titrating oxalic acid with permanganate?

  • Yes, methyl orange is required
  • Yes, phenolphthalein is required
  • No, permanganate is self-indicating by its purple color
  • Yes, iodine is required as an indicator

Correct Answer: No, permanganate is self-indicating by its purple color

Q8. How many electrons are transferred per molecule of oxalic acid (H2C2O4) when it is oxidized to CO2?

  • 1 electron
  • 2 electrons
  • 4 electrons
  • 5 electrons

Correct Answer: 2 electrons

Q9. To prepare 1.00 L of 0.100 M oxalic acid solution, what mass of H2C2O4·2H2O (molar mass 126.07 g mol−1) is required?

  • 12.61 g
  • 126.07 g
  • 6.303 g
  • 25.21 g

Correct Answer: 12.61 g

Q10. What is the equivalent weight of oxalic acid dihydrate in redox reactions (g per eq)?

  • 126.07 g eq−1
  • 63.035 g eq−1
  • 42.02 g eq−1
  • 31.52 g eq−1

Correct Answer: 63.035 g eq−1

Q11. Which gas is evolved as a product during the oxidation of oxalic acid by permanganate?

  • Oxygen (O2)
  • Carbon dioxide (CO2)
  • Carbon monoxide (CO)
  • Hydrogen (H2)

Correct Answer: Carbon dioxide (CO2)

Q12. Why must glassware and the burette be rinsed with the titrant solution before titration?

  • To sterilize the equipment
  • To ensure temperature equilibrium
  • To avoid dilution errors and ensure concentration consistency
  • To neutralize any residual acid

Correct Answer: To avoid dilution errors and ensure concentration consistency

Q13. Which of the following impurities in reagents would most directly interfere with accurate standardization of KMnO4 using oxalic acid?

  • Non-reactive salts like NaCl
  • Reducing impurities such as iron(II) or organic reducing agents
  • Trace amounts of noble gases
  • Slightly acidic pH from CO2 absorption

Correct Answer: Reducing impurities such as iron(II) or organic reducing agents

Q14. If 25.00 mL of 0.0200 M KMnO4 is completely reacted, what volume of 0.100 M H2C2O4 is required? (Use stoichiometry 5 H2C2O4 : 2 KMnO4)

  • 6.25 mL
  • 12.50 mL
  • 25.00 mL
  • 50.00 mL

Correct Answer: 12.50 mL

Q15. Which practical step is critical to obtain a reproducible endpoint when standardizing KMnO4 with oxalic acid?

  • Add titrant rapidly near the endpoint
  • Perform titration at room temperature without heating
  • Maintain constant elevated temperature and add titrant slowly near endpoint
  • Use distilled water to acidify the solution

Correct Answer: Maintain constant elevated temperature and add titrant slowly near endpoint

Q16. How does the presence of chloride ions (Cl−) in the reaction mixture affect KMnO4 titration?

  • Chloride has no effect
  • Chloride oxidizes to chlorine, causing side reactions and errors
  • Chloride reduces permanganate, increasing accuracy
  • Chloride forms a complex that stabilizes KMnO4

Correct Answer: Chloride oxidizes to chlorine, causing side reactions and errors

Q17. What is the visible sign that indicates the endpoint in a permanganate titration of oxalic acid?

  • Solution turns clear and colorless
  • Permanent faint pink color persists for about 20–30 seconds
  • Solution emits gas bubbles vigorously
  • Precipitate forms

Correct Answer: Permanent faint pink color persists for about 20–30 seconds

Q18. Which property is NOT desirable in a primary standard used to prepare a standard oxalic acid solution?

  • High purity and known composition
  • Non-hygroscopic and stable to air
  • Complex, variable composition
  • High equivalent weight for reduced weighing error

Correct Answer: Complex, variable composition

Q19. How should a prepared oxalic acid standard solution be stored to maintain concentration?

  • In an open beaker at room temperature
  • In a labeled, tightly stoppered amber glass bottle away from light and heat
  • In a clear plastic bottle exposed to sunlight
  • Mixed with KMnO4 for stabilization

Correct Answer: In a labeled, tightly stoppered amber glass bottle away from light and heat

Q20. When converting molarity of oxalic acid solution to normality for redox titration with permanganate, what is the conversion factor?

  • Normality = Molarity × 1
  • Normality = Molarity × 2
  • Normality = Molarity × 0.5
  • Normality = Molarity × 5

Correct Answer: Normality = Molarity × 2

Q21. Which procedural error will most likely lead to a lower calculated concentration of the standardized KMnO4?

  • Rinsing the burette with the titrant before filling
  • Allowing solution to cool below the titration temperature before reading endpoint
  • Adding titrant dropwise near the endpoint
  • Accurately weighing primary standard

Correct Answer: Allowing solution to cool below the titration temperature before reading endpoint

Q22. Which statement correctly describes the redox half-reaction for permanganate in acidic medium?

  • MnO4− + 8 H+ + 5 e− → Mn2+ + 4 H2O
  • MnO4− + 2 H+ + 3 e− → MnO2 + H2O
  • MnO4− + e− → MnO4 2−
  • MnO4− + 4 H+ + 3 e− → MnO2 + 2 H2O

Correct Answer: MnO4− + 8 H+ + 5 e− → Mn2+ + 4 H2O

Q23. If oxalic acid is used as a primary standard for acid-base titration with NaOH, how many moles of NaOH react with one mole of H2C2O4?

  • 1 mole NaOH per mole H2C2O4
  • 2 moles NaOH per mole H2C2O4
  • 0.5 mole NaOH per mole H2C2O4
  • 3 moles NaOH per mole H2C2O4

Correct Answer: 2 moles NaOH per mole H2C2O4

Q24. Which of the following best explains why KMnO4 solutions are standardized frequently?

  • KMnO4 concentration is constant over time
  • KMnO4 is a primary standard and does not need standardization
  • KMnO4 can decompose and its concentration changes due to contamination and self-decomposition
  • Permanganate is unaffected by light or organic contaminants

Correct Answer: KMnO4 can decompose and its concentration changes due to contamination and self-decomposition

Q25. During preparation of standard oxalic acid solution, why must the weighed oxalic acid be transferred quantitatively to the volumetric flask?

  • To avoid thermal shock to the flask
  • To ensure the entire weighed mass is present so concentration is accurate
  • To prevent gas evolution
  • To change the pH of the solution

Correct Answer: To ensure the entire weighed mass is present so concentration is accurate

Q26. Which safety precaution is most appropriate when handling solid oxalic acid in the lab?

  • No special precautions are required
  • Use gloves and eye protection because oxalic acid is toxic and corrosive
  • Only wear a lab coat; oxalic acid is non-toxic
  • Work without ventilation to avoid dilution

Correct Answer: Use gloves and eye protection because oxalic acid is toxic and corrosive

Q27. What is the main reason for boiling or heating the reaction mixture just before titration when using oxalic acid and permanganate?

  • To concentrate oxalic acid by evaporation
  • To speed up reaction kinetics and ensure complete oxidation during titration
  • To decompose permanganate
  • To remove dissolved CO2

Correct Answer: To speed up reaction kinetics and ensure complete oxidation during titration

Q28. If a student records an endpoint color that disappears rapidly after stirring stops, what is the likely cause?

  • Endpoint was correct and stable
  • Insufficient mixing or endpoint not reached; transient color indicates endpoint overshoot or incomplete reaction
  • Solutions are too concentrated
  • Glassware was not clean

Correct Answer: Insufficient mixing or endpoint not reached; transient color indicates endpoint overshoot or incomplete reaction

Q29. How many moles of electrons are exchanged when 1 mole of KMnO4 is reduced to Mn2+ in acidic medium?

  • 1 mole of electrons
  • 3 moles of electrons
  • 5 moles of electrons
  • 2 moles of electrons

Correct Answer: 5 moles of electrons

Q30. Which calculation is correct for determining molarity of KMnO4 if 0.1250 g of pure H2C2O4·2H2O (126.07 g mol−1) required 20.00 mL of KMnO4 to reach endpoint? (Use reaction stoichiometry 5 H2C2O4 : 2 KMnO4)

  • Moles H2C2O4 = 0.1250/126.07 = 0.000991 mol; moles KMnO4 = (2/5)×0.000991 = 0.0003964; M = 0.0003964/0.02000 = 0.01982 M
  • Moles H2C2O4 = 0.1250/126.07 = 0.00991 mol; M = 0.00991/0.02000 = 0.4955 M
  • Moles H2C2O4 = 0.1250/126.07 = 0.000991 mol; moles KMnO4 = (5/2)×0.000991; M = 0.00248/0.02000 = 0.124 M
  • Moles H2C2O4 = 0.1250/126.07 = 0.000991 mol; M = 0.000991/0.02000 = 0.0496 M

Correct Answer: Moles H2C2O4 = 0.1250/126.07 = 0.000991 mol; moles KMnO4 = (2/5)×0.000991 = 0.0003964; M = 0.0003964/0.02000 = 0.01982 M

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