Over 100 Short Q&A for Redox and Acid-Base Titrations (Class 12 NCERT Lab Manual)

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Redox & Acid-Base Titrations • Class 12 NCERT Lab Manual

Class 12 Practical Exam / JEE

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105 Short Q&A Facts

Part 1: Basic Volumetric Concepts
1. Q1: What is Volumetric Analysis?
   A: A quantitative analysis technique based on measuring the volume of a solution of known concentration required to react completely with a solution of unknown concentration.
2. Q2: Define Titration.
   A: The process of adding the solution from the burette to the solution in the conical flask until the reaction is complete.
3. Q3: Define Titre.
   A: The volume of solution taken from the burette during the titration.
4. Q4: What is the Endpoint of a titration?
   A: The point where the indicator changes color, signaling the completion of the reaction.
5. Q5: What is the Equivalence Point of a titration?
   A: The point where the moles (or equivalents) of the reactant from the burette exactly equal the moles (or equivalents) of the reactant in the flask.
6. Q6: What is the ideal relationship between the endpoint and the equivalence point?
   A: They should be as close as possible.
7. Q7: Define a Standard Solution.
   A: A solution whose concentration is accurately known.
8. Q8: Define a Primary Standard.
   A: A substance that can be obtained in high purity, is stable, and has a known chemical composition (used to make standard solutions directly).
9. Q9: Give two examples of Primary Standards used in titrations.
   A: Oxalic Acid (Acid-Base) and Anhydrous Na₂CO₃ (Acid-Base).
10. Q10: Define a Secondary Standard.
    A: A substance that cannot be obtained in high purity or is unstable, and must be standardized against a primary standard (e.g., NaOH, KMnO₄).
Part 2: Acid-Base Titrations (Acidimetry and Alkalimetry)
11. Q11: What type of reaction is the basis of an Acid-Base titration?
    A: Neutralization Reaction.
12. Q12: What is the fundamental relationship used at the equivalence point in acid-base titration?
    A: N₁V₁ = N₂V₂ (or Moles_A = Moles_B for 1:1 reaction).
13. Q13: What is the purpose of phenolphthalein indicator?
    A: To indicate the endpoint in titrations involving a strong base (pH range 8.3-10.5).
14. Q14: What color change does phenolphthalein show in a basic solution?
    A: Colorless to Pink.
15. Q15: What color change does phenolphthalein show in an acidic solution?
    A: Colorless to Colorless.
16. Q16: What is the purpose of methyl orange indicator?
    A: To indicate the endpoint in titrations involving a strong acid (pH range 3.1-4.4).
17. Q17: What color change does methyl orange show in an acidic solution?
    A: Yellow to Red.
18. Q18: Which indicator is suitable for the titration of a Strong Acid vs. Strong Base (HCl vs NaOH)?
    A: Phenolphthalein or Methyl Orange (endpoint pH ≈ 7, both work).
19. Q19: Which indicator is suitable for the titration of a Weak Acid vs. Strong Base (Oxalic vs NaOH)?
    A: Phenolphthalein (endpoint pH > 7, basic range).
20. Q20: Which indicator is suitable for the titration of a Strong Acid vs. Weak Base (HCl vs Na₂CO₃)?
    A: Methyl Orange (endpoint pH < 7, acidic range).
21. Q21: Why is NaOH considered a secondary standard?
    A: It is hygroscopic (absorbs moisture) and absorbs CO₂ from the air, making its concentration unstable.
22. Q22: Why is Oxalic Acid considered a primary standard?
    A: It is stable, non-hygroscopic, and available in high purity.
23. Q23: Why is the conical flask rinsed only with water before titration?
    A: To avoid changing the moles of the substance being analyzed (water doesn't add moles of solute).
24. Q24: Why is the burette rinsed with the titrant solution before titration?
    A: To ensure the concentration of the liquid dispensed is not diluted by residual water.
25. Q25: What is the common source of error when reading the burette?
    A: Parallax error (not reading at eye level).
26. Q26: What part of the liquid meniscus should be read in a burette for colorless solutions?
    A: The lower meniscus.
Part 3: Redox Titrations (Permanganometry)
27. Q27: What type of reaction is the basis of a Redox titration?
    A: Oxidation-Reduction (electron transfer).
28. Q28: What is the primary oxidizing agent used in Permanganometry experiments?
    A: Potassium Permanganate (KMnO₄).
29. Q29: Why is KMnO₄ considered a secondary standard?
    A: It is difficult to obtain in high purity and may contain MnO₂ impurity.
30. Q30: Against which primary standard is KMnO₄ usually standardized?
    A: Oxalic Acid or Mohr's Salt (Ferrous Ammonium Sulphate).
31. Q31: Why must KMnO₄ titrations be carried out in an acidic medium?
    A: KMnO₄ is the strongest oxidizing agent in acidic solution (MnO₄^- → Mn²⁺).
32. Q32: What is the reducing agent when KMnO₄ reacts with Oxalic Acid?
    A: Oxalate ion (C₂O₄^2-).
33. Q33: What are the oxidation products of Oxalic Acid in this titration?
    A: Carbon Dioxide (CO₂) and Water (C₂O₄^2- → 2CO₂).
34. Q34: What is the oxidation number change for Mn in acidic medium (KMnO₄)?
    A: From +7 to +2 (n-factor = 5).
35. Q35: What is the oxidation number change for Carbon in Oxalic Acid?
    A: From +3 to +4 (n-factor per C₂O₄^2- unit = 2).
36. Q36: What is the self-indicator in the KMnO₄ vs. Oxalic Acid titration?
    A: KMnO₄ itself.
37. Q37: What is the color change at the endpoint of the KMnO₄ titration?
    A: Colorless to Permanent Pale Pink.
38. Q38: Why is the solution heated to 60-70°C during the KMnO₄ vs Oxalic Acid titration?
    A: To increase the reaction rate, as the reaction is slow at room temperature.
39. Q39: What is the catalyst for the KMnO₄ vs Oxalic Acid titration?
    A: Mn²⁺ ions (Autocatalysis).
40. Q40: What is the reducing agent when KMnO₄ reacts with Mohr's Salt?
    A: Ferrous ion (Fe²⁺).
41. Q41: What is the oxidation product of the Fe²⁺ ion?
    A: Ferric ion (Fe³⁺) (Fe²⁺ → Fe³⁺).
42. Q42: What is the n-factor for Fe²⁺ in this reaction?
    A: 1 (Change from +2 to +3).
43. Q43: Why is FeSO₄ (Ferrous Sulphate) preferred as a secondary standard over FeSO₄ alone?
    A: Mohr's Salt (FAS) is more stable and does not readily oxidize in air.
44. Q44: What is the disadvantage of using KMnO₄ in a basic medium?
    A: It reduces to brown/black MnO₂, making the endpoint difficult to detect.
Part 4: Calculation and Equivalence Concepts
45. Q45: Define Normality (N).
    A: Number of gram equivalents of solute per litre of solution.
46. Q46: Define Equivalent Mass of an acid.
    A: Molar Mass / Basicity (number of replaceable H⁺ ions).
47. Q47: Define Equivalent Mass of a base.
    A: Molar Mass / Acidity (number of replaceable OH^- ions).
48. Q48: Define Equivalent Mass of an oxidizing/reducing agent.
    A: Molar Mass / Change in Oxidation Number (n-factor).
49. Q49: What is the relationship between Normality and Molarity?
    A: N = M × n-factor.
50. Q50: What is the equivalent mass of HCl?
    A: 36.5 g/eq (Molar Mass / 1).
51. Q51: What is the equivalent mass of H₂SO₄ in a neutralization reaction?
    A: 49.0 g/eq (Molar Mass / 2).
52. Q52: What is the equivalent mass of KMnO₄ in an acidic medium?
    A: ≈ 31.6 g/eq (Molar Mass / 5).
53. Q53: What is the equivalent mass of Mohr’s Salt in a redox titration?
    A: Molar Mass / 1 (Only Fe²⁺ is oxidized).
54. Q54: How many grams of NaOH (mol mass 40) are in 1 L of 1 N solution?
    A: 40 g (Eq Mass = 40).
55. Q55: How many grams of H₂SO₄ (mol mass 98) are in 1 L of 1 N solution?
    A: 49 g (Eq Mass = 49).
Part 5: Indicators and Endpoint Detection
56. Q56: What is an Acid-Base Indicator?
    A: A weak organic acid or base whose undissociated form has a different color than its ionic form.
57. Q57: What is the mechanism by which an indicator changes color?
    A: The shift in the indicator's dissociation equilibrium due to the change in pH.
58. Q58: What pH range should an indicator's color change cover for accurate results?
    A: The color change should occur within the steep portion of the titration curve.
59. Q59: Why is the endpoint of CH₃COOH vs NaOH not sharp?
    A: The pH change around the equivalence point is less steep (buffering action of the salt).
60. Q60: Can a common indicator (like phenolphthalein) be used in a redox titration?
    A: No, redox indicators are required, or the titrant must be self-indicating (KMnO₄).
61. Q61: Give an example of an external indicator used in a redox titration.
    A: Starch solution (used for I₂ titrations).
62. Q62: What color complex does Starch form with I₂?
    A: Deep blue.
63. Q63: What is a Concordant Reading?
    A: Titre values that agree closely (usually within ±0.1 mL of each other).
64. Q64: What is the major error when the color change is observed only after a delay of 30 s?
    A: Overshooting the endpoint (endpoint should be recorded at the first permanent color change).
Part 6: Practical Precautions and Calculations
65. Q65: What is the correct way to add the titrant near the endpoint?
    A: Drop by drop, with continuous swirling.
66. Q66: What is the purpose of placing a white tile/paper under the conical flask?
    A: To clearly visualize the color change at the endpoint.
67. Q67: What substance should be avoided near a KMnO₄ titration?
    A: Rubber (as KMnO₄ oxidizes rubber), hence glass stopcocks are preferred.
68. Q68: Why must the burette tip be free of air bubbles before starting?
    A: Air bubbles introduce volume errors in the titre reading.
69. Q69: What is the final step in standardizing a secondary standard solution?
    A: Calculate the exact molarity/normality using the titration reading and the known concentration of the primary standard.
70. Q70: How is the percentage purity of a sample determined using titration data?
    A: % Purity = (Mass of pure substance / Mass of impure sample) × 100.
71. Q71: If V₁ L of M₁ M acid reacts with V₂ L of M₂ M base (1:1 mole ratio), what is the relationship at the endpoint?
    A: M₁V₁ = M₂V₂.
Part 7: More on Permanganometry and Oxalic Acid
72. Q72: What is the full name of Mohr's Salt?
    A: Ferrous Ammonium Sulphate (FeSO₄·(NH₄)₂SO₄·6H₂O).
73. Q73: What is the chemical formula of Oxalic Acid dihydrate?
    A: H₂C₂O₄·2H₂O.
74. Q74: What happens if H₂SO₄ is not added to the conical flask in the KMnO₄ titration?
    A: The reaction will take place in a neutral/basic medium, reducing KMnO₄ to MnO₂ (brown ppt), obscuring the endpoint.
75. Q75: Why is the initial addition of KMnO₄ to hot oxalic acid slow?
    A: The initial reaction is slow; Mn²⁺ (autocatalyst) has not yet formed in sufficient concentration.
76. Q76: Why must the KMnO₄ solution be stored in dark bottles?
    A: Light decomposes KMnO₄ into MnO₂, affecting its concentration.
77. Q77: What precaution is necessary regarding HCl when standardizing KMnO₄?
    A: HCl cannot be used for acidification because KMnO₄ will oxidize Cl^- to Cl₂.
78. Q78: What is the standard concentration used for KMnO₄ solutions in the lab manual?
    A: Usually M/20 or N/10.
Part 8: Additional Titration Concepts
79. Q79: What is a Double Titration?
    A: Titrating a mixture of two components (e.g., NaOH and Na₂CO₃) using two different indicators (phenolphthalein and methyl orange).
80. Q80: In the double titration of NaOH and Na₂CO₃ with HCl, what happens at the phenolphthalein endpoint?
    A: NaOH is completely neutralized, and Na₂CO₃ is converted to NaHCO₃.
81. Q81: What happens between the phenolphthalein endpoint and the methyl orange endpoint?
    A: NaHCO₃ is converted to H₂CO₃.
82. Q82: What is the purpose of adding 1 g of KI in the titration of Cu²⁺?
    A: I^- is oxidized by Cu²⁺ to I₂ (Iodometry) which is then titrated.
83. Q83: Why should the titration of iodine (I₂) be done as soon as it is prepared?
    A: I₂ is volatile and its concentration will change.
84. Q84: Which indicator is used in Iodometric titrations?
    A: Starch solution.
85. Q85: Why is starch indicator added only when the I₂ color is faint yellow?
    A: Starch forms a stable complex with high I₂ concentration, making the endpoint inaccurate.
86. Q86: What is the relationship between the Equivalent Mass of Na₂S₂O₃ and its Molar Mass?
    A: 1:1 (Na₂S₂O₃ → Na₂S₄O₆ in the reaction).
Part 9: Error Analysis and Standardisation
87. Q87: What is the difference between a systematic error and a random error?
    A: Systematic errors are constant (e.g., faulty balance); Random errors vary (e.g., reading meniscus).
88. Q88: What is the main source of random error in titration?
    A: Endpoint detection and meniscus reading.
89. Q89: What is the standard pH for a neutral solution at 25°C?
    A: 7.0.
90. Q90: Why is standardizing secondary solutions crucial?
    A: To determine their exact concentration due to their unstable or impure nature.
91. Q91: What is the primary use of a pipette in titration?
    A: To accurately measure a fixed volume of the solution into the conical flask.
92. Q92: What is the primary use of a volumetric flask?
    A: To accurately prepare a solution of a specific volume and known concentration.
93. Q93: Why should the pipette never be blown out to dispense the final drop?
    A: The pipette is calibrated to deliver the volume only when the tip is allowed to drain freely.
94. Q94: What is the term for the final, permanent color observed?
    A: Permanent Endpoint.
Part 10: Final Revision Points
95. Q95: What is the n-factor of H₃PO₄ in a titration with NaOH?
    A: 3 (Tribasic acid).
96. Q96: What is the n-factor of H₃BO₃ (Boric Acid)?
    A: 1 (Monobasic acid, acts as a Lewis acid).
97. Q97: What is the equivalent mass of Na₂CO₃ in a titration against HCl?
    A: Molar Mass / 2 (releases 2 Na⁺ or requires 2 H⁺).
98. Q98: Why is an acidic medium preferred for KMnO₄ titrations?
    A: The reduction is cleanest and fastest, and the endpoint is sharp (colorless to pink).
99. Q99: What is the chemical that causes the pale pink color at the endpoint of a KMnO₄ titration?
    A: Excess MnO₄^- ions (the reducing agent is depleted).
100. Q100: Why must the conical flask be swirled continuously during titration?
     A: To ensure complete and homogeneous mixing of the reactants.
101. Q101: If the normality is 2 N and the volume is 20 mL, how many equivalents are present?
     A: 0.04 equivalents (2 N × 0.02 L).
102. Q102: What must be done if the initial burette reading is below the 0.0 mL mark?
     A: Record the initial reading as is, but ensure the volume is sufficient for the entire titration.
103. Q103: If the pH changes from 4 to 10 near the equivalence point, what acid/base types are involved?
     A: Strong Acid vs. Strong Base (large pH jump).
104. Q104: What is the term for the method where the analyte is added to the burette?
     A: Direct Titration.
105. Q105: What is the common catalyst added in the standardization of KMnO₄ against Mohr’s Salt?
     A: Dilute H₂SO₄ (provides the acidic medium).

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