A comprehensive question bank for Qualitative and Quantitative Organic Analysis - PART-B

Advanced Viva Voce Questions (Part 2)

Deep dive into Techniques, Safety, and Troubleshooting

More on Qualitative Analysis (Separations)

I. Preliminary Tests & Observation

• Q1: What does the "Ignition Test" tell you about an unknown solid?
  Answer: Burning on a spatula: A sooty (black smoke) flame usually indicates an Aromatic compound (high carbon content). A non-sooty flame indicates an Aliphatic compound.
• Q2: What is the "Beilstein Test"?
  Answer: A rapid test for Halogens. A copper wire is heated with the compound; a green/blue flame indicates the presence of Chlorine, Bromine, or Iodine (formation of volatile copper halides).
• Q3: Why do some acids (like Salicylic Acid) give a violet color with FeCl₃?
  Answer: Because they contain a phenolic -OH group in addition to the carboxylic acid group. It acts as both an acid and a phenol.

II. Chromatography (TLC)

• Q4: Define the R_f value in Thin Layer Chromatography (TLC).
  Answer: Retardation Factor (R_f) = (Distance traveled by solute) / (Distance traveled by solvent front). It is always less than 1.
• Q5: If two spots on a TLC plate have the same R_f value, are they definitely the same compound?
  Answer: Not definitely. They might just have similar polarity in that specific solvent system. You must check with a different solvent system (co-chromatography) to be sure.

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More on Quantitative Analysis (Estimations)

I. Standards & Solutions

• Q6: What is the difference between a Primary and Secondary Standard?
  Answer:
  • Primary Standard: Pure, stable, non-hygroscopic substance used to make a solution of known concentration directly (e.g., Potassium Dichromate, Oxalic Acid).
  • Secondary Standard: A solution whose concentration changes over time (like NaOH or KMnO₄) and must be standardized against a primary standard.
• Q7: Why can't we weigh solid NaOH to make a standard solution directly?
  Answer: NaOH is hygroscopic (absorbs moisture) and reacts with atmospheric CO₂ to form carbonates. You can never weigh it accurately.

II. Iodometric Titrations (if applicable to syllabus extensions)

• Q8: What is the difference between Iodometry and Iodimetry?
  Answer:
  • Iodimetry: Direct titration using a standard Iodine solution.
  • Iodometry: Indirect titration where Iodine is liberated in the reaction (e.g., KI + Oxidizing agent) and then titrated with Sodium Thiosulfate.
• Q9: Why is Starch indicator added near the end of the titration, not at the beginning?
  Answer: If added too early (when iodine concentration is high), starch forms a stable complex with iodine that is difficult to break, leading to a fading endpoint and error. We wait until the solution is pale yellow (straw color).

III. Error Analysis

• Q10: What is "Parallax Error"?
  Answer: An error in reading the burette caused by the eye not being at the same level as the meniscus. (Read lower meniscus for colorless liquids, upper for colored).
• Q11: Does using a larger sample size increase or decrease accuracy?
  Answer: Generally, it increases accuracy because it reduces the relative percentage error of weighing and titration volume reading.

💡 Examiner's Tip: If you are unsure of an answer, never stay silent. Say: "I recall reading about this, but I cannot remember the exact term right now. May I explain the concept instead?" This shows understanding even if memory fails.