JEE Chemistry Purification And Characterisation Of Organic Compounds MCQs Set 03

Multiple Choice Questions (MCQs) for JEE Chemistry: Purification And Characterisation Of Organic Compounds

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Practice Purification And Characterisation Of Organic Compounds MCQs for JEE Chemistry

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Multiple Choice Questions

Question 1: In the Kjeldahl estimation of nitrogen, 1.4 g of an organic compound was digested and the evolved ammonia was absorbed in 60 mL of M/10 H₂SO₄. The unreacted acid required 20 mL of M/10 NaOH for complete neutralisation. The percentage of nitrogen in the compound is:
(a) 6%
(b) 10%
(c) 3%
(d) 5%
Show Answer & Explanation

Answer: (b) 10%

Explanation:
1. Acid taken = 60 × 0.1 × 2 = 12 milliequivalents; acid left = 20 × 0.1 = 2 milliequivalents.
2. Acid used by NH₃ = 10 milliequivalents, so NH₃ = 10 mmol and N = 10 × 14 = 140 mg.
3. % N = 0.140/1.4 × 100 = 10%.

Teacher's Note:
1. Percentage of N = 1.4 × (milliequivalents of acid neutralised by NH₃) / mass of compound in g.

Question 2: 29.5 mg of an organic compound containing nitrogen was digested by Kjeldahl's method and the evolved ammonia was absorbed in 20 mL of 0.1 M HCl. The excess acid required 15 mL of 0.1 M NaOH for complete neutralisation. The percentage of nitrogen in the compound is:
(a) 59.0
(b) 47.4
(c) 23.7
(d) 29.5
Show Answer & Explanation

Answer: (c) 23.7

Explanation:
1. HCl taken = 20 × 0.1 = 2 mmol; HCl left = 15 × 0.1 = 1.5 mmol.
2. HCl used = 0.5 mmol = NH₃ = N atoms, mass of N = 0.5 × 14 = 7 mg.
3. % N = 7/29.5 × 100 = 23.7.

Teacher's Note:
1. Moles of N = moles of acid neutralised by ammonia.

Question 3: The compound formed in the positive test for nitrogen with the Lassaigne extract of an organic compound is:
(a) Fe₄[Fe(CN)₆]₃
(b) Na₃[Fe(CN)₆]
(c) Fe(CN)₃
(d) Na₄[Fe(CN)₅NOS]
Show Answer & Explanation

Answer: (a) Fe₄[Fe(CN)₆]₃

Explanation:
1. Nitrogen is converted to NaCN, which forms Na₄[Fe(CN)₆] with FeSO₄.
2. On adding FeCl₃, Prussian blue, Fe₄[Fe(CN)₆]₃, is formed.

Teacher's Note:
1. Prussian blue = positive test for nitrogen.

Question 4: Which of the following will not give Lassaigne's test for nitrogen?
(a) NH₂NH₂
(b) C₆H₅N=NC₆H₅
(c) CH₃CONH₂
(d) CH₃C≡N
Show Answer & Explanation

Answer: (a) NH₂NH₂

Explanation:
1. In Lassaigne's test N must combine with carbon and sodium to give NaCN.
2. Hydrazine contains no carbon, so NaCN cannot form.

Teacher's Note:
1. Compounds with no carbon (hydrazine) fail the Lassaigne test for nitrogen.

Question 5: Sulphur in an organic compound is converted to Na₂S in the Lassaigne fusion. Na₂S can be detected by: (I) lead acetate, (II) acetic acid, (III) sodium nitroprusside.
(a) (I) and (II)
(b) (I) and (III)
(c) (III) only
(d) (I), (II) and (III)
Show Answer & Explanation

Answer: (b) (I) and (III)

Explanation:
1. Lead acetate gives a black precipitate of PbS.
2. Sodium nitroprusside gives a violet or purple colour.
3. Acetic acid is only used to acidify the extract before the lead acetate test and is not a detecting reagent.

Teacher's Note:
1. S²⁻ is detected by lead acetate (black PbS) or nitroprusside (purple).

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Practice MCQs for JEE Chemistry Purification And Characterisation Of Organic Compounds

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