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Qualitative detection of nitrogen, sulphur, phosphorus and halogens: NEET MCQs with answers

90 questions on this topic of Purification and Characterisation of Organic Compounds (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. An organic compound containing sulfur is subjected to sodium fusion. Which of the following species in the resulting extract is directly responsible for forming a black precipitate with lead acetate?

  1. SO₄²⁻
  2. S₂O₃²⁻
  3. SCN⁻
  4. S²⁻
Show answer and explanation

Answer: (D) S²⁻

Fusion of sulfur-containing organic compounds with sodium yields sodium sulphide (Na₂S). The sulfide ion (S²⁻) reacts with Pb²⁺ from lead acetate to precipitate black lead sulphide (PbS).

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2. When an organic compound containing both nitrogen and sulphur is fused with an excess of sodium metal, what species are formed in the Lassaigne's extract, and what colour is observed on adding neutral FeCl₃?

  1. NaSCN is formed; blood-red colouration is observed
  2. NaSCN and Na₂SO₄ are formed; violet colouration is observed
  3. NaCN and Na₂S are formed; Prussian blue colour is observed without blood-red colour
  4. NaCN and Na₂S are formed; no colouration is observed
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Answer: (C) NaCN and Na₂S are formed; Prussian blue colour is observed without blood-red colour

When sodium is taken in excess during fusion, any thiocyanate initially formed decomposes into sodium cyanide and sodium sulphide: NaSCN + 2Na → NaCN + Na₂S. Consequently, the extract gives a Prussian blue test for nitrogen with FeSO₄/FeCl₃ rather than the blood-red [Fe(SCN)]²⁺ complex.

hard

3. A Lassaigne's extract from a compound containing carbon, hydrogen, and nitrogen was boiled with FeSO₄ and acidified with dilute H₂SO₄. The number of moles of NaCN required per mole of Prussian blue, Fe₄[Fe(CN)₆]₃, formed is:

  1. 18
  2. 12
  3. 6
  4. 24
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Answer: (A) 18

Prussian blue has the stoichiometry Fe₄[Fe(CN)₆]₃. Each formula unit contains three hexacyanoferrate(II) units, which requires 3 × 6 = 18 cyanide groups. Therefore, 18 moles of NaCN are consumed to produce one mole of Fe₄[Fe(CN)₆]₃.

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4. An organic compound containing carbon, hydrogen, nitrogen, and sulfur is fused with sodium. The resulting extract gives a blood-red colouration when treated with neutral FeCl₃. What happens if this same fusion is carried out with an excess of sodium metal?

  1. The blood-red colouration becomes significantly more intense
  2. White precipitate of FeS is obtained upon adding acid
  3. A yellow precipitate of iron(III) thiocyanate is formed
  4. Prussian blue precipitate forms instead of the blood-red colouration upon treatment with FeSO₄ and acid
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Answer: (D) Prussian blue precipitate forms instead of the blood-red colouration upon treatment with FeSO₄ and acid

When sodium is in stoichiometric deficit or equal amounts, NaSCN forms, which reacts with Fe³⁺ to give blood-red [Fe(SCN)]²⁺. However, with an excess of sodium, NaSCN is decomposed into NaCN and Na₂S. The NaCN then responds normally to give the Prussian blue test with FeSO₄ and acid.

easy

5. Given below are two statements: Statement I: Carbon and nitrogen present in an organic compound fuse together with sodium metal to form covalent cyanide molecules. Statement II: The purpose of sodium fusion is to convert elements present in covalent form in an organic compound into water-soluble ionic forms. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (C) Statement I is incorrect but Statement II is correct

Statement I is incorrect because carbon and nitrogen fuse with sodium to form ionic sodium cyanide (NaCN), not covalent molecules. Statement II is correct as the fundamental objective of Lassaigne's test is converting covalent organic heteroatoms into ionic, water-soluble sodium salts (NaCN, Na₂S, NaX).

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6. An organic compound containing carbon, hydrogen, nitrogen, and sulphur is fused with a limited amount of sodium metal. The extract is divided into two portions. Portion 1 gives a blood-red colour with FeCl₃. If Portion 2 is treated directly with sodium nitroprusside solution, what will be observed?

  1. A brilliant purple/violet colour appears
  2. No purple/violet colour appears because free sulphide ions are absent
  3. A black precipitate is immediately formed
  4. A canary yellow precipitate settles out
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Answer: (B) No purple/violet colour appears because free sulphide ions are absent

When sodium is used in limited amount, carbon, nitrogen, and sulphur combine into NaSCN rather than forming separate NaCN and Na₂S. Because free S²⁻ ions are not formed, the extract fails to give the characteristic purple/violet colour with sodium nitroprusside.

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7. Which of the following organic compounds will produce an intense blood-red colouration with FeCl₃ after sodium fusion, when sodium metal is used in a strictly limited (deficient) quantity?

  1. Thiourea (NH₂CSNH₂)
  2. Urea (NH₂CONH₂)
  3. Sulphanilic acid treated with excess sodium
  4. Glycine (H₂NCH₂COOH)
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Answer: (A) Thiourea (NH₂CSNH₂)

Thiourea contains both nitrogen and sulphur along with carbon. When fused with a limited amount of sodium metal, it forms sodium thiocyanate (NaSCN). Upon acidification and addition of Fe³⁺ ions, NaSCN produces the characteristic blood-red complex [Fe(SCN)]²⁺.

easy

8. Which of the following mixtures of reagents can be used to acidify the sodium fusion extract prior to testing for sulfur using lead acetate solution?

  1. Dilute hydrochloric acid
  2. Acetic acid
  3. Dilute sulphuric acid
  4. Concentrated nitric acid
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Answer: (B) Acetic acid

Acetic acid is used to acidify the Lassaigne's extract in the lead acetate test. Mineral acids like dilute H₂SO₄ or dilute HCl are avoided because lead sulphate or lead chloride would precipitate, obscuring the black lead sulphide precipitate.

easy

9. Given below are two statements: Statement I: Fluorine present in an organic compound cannot be reliably detected by the silver nitrate test via Lassaigne's extract. Statement II: Silver fluoride (AgF) is readily soluble in water and does not form a precipitate. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Fluorine forms sodium fluoride (NaF) during fusion. Upon addition of AgNO₃, silver fluoride (AgF) is produced, which is highly soluble in water and does not precipitate. Hence, Lassaigne's test with AgNO₃ cannot detect fluorine.

easy

10. Given below are two statements: Statement I: Hydroxylamine hydrochloride (NH₂OH·HCl) does not give Prussian blue colour in Lassaigne's test for nitrogen. Statement II: Organic compounds containing nitrogen must also contain carbon to form cyanide ions during fusion with sodium metal. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Hydroxylamine hydrochloride lacks carbon in its molecular formula. During sodium fusion, cyanide ions (CN⁻) cannot be formed because carbon is absent, so it gives a negative Lassaigne's test for nitrogen. Both statements are correct, and Statement II explains why.

easy

11. In the detection of sulphur using the sodium nitroprusside test, the appearance of a purple or violet colouration is due to the formation of which of the following coordination complexes?

  1. [Fe(CN)₅(NOS)]⁴⁻
  2. [Fe(CN)₅(NOS)]³⁻
  3. [Fe(CN)₅(SO₄)]⁴⁻
  4. [Fe(CN)₆]⁴⁻
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Answer: (A) [Fe(CN)₅(NOS)]⁴⁻

Sulphide ion (S²⁻) from the sodium fusion extract reacts with sodium nitroprusside, [Fe(CN)₅(NO)]²⁻, in alkaline medium to form a purple-coloured complex, sodium thionitroprusside, Na₄[Fe(CN)₅(NOS)]. The complex ion is [Fe(CN)₅(NOS)]⁴⁻.

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12. In the Lassaigne's test for nitrogen, freshly prepared FeSO₄ solution is added to the alkaline fusion extract and heated. The primary reason for using 'freshly prepared' FeSO₄ is:

  1. Freshly prepared FeSO₄ contains dissolved oxygen which is required to oxidise NaCN
  2. Old FeSO₄ precipitates out completely as metallic iron
  3. Fe²⁺ ions are converted to insoluble FeS on standing in air
  4. Old FeSO₄ solutions get oxidised by air to basic ferric sulphate, which interferes with the test
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Answer: (D) Old FeSO₄ solutions get oxidised by air to basic ferric sulphate, which interferes with the test

Ferrous sulphate gets readily oxidised by atmospheric oxygen to basic ferric sulphate [Fe(OH)SO₄], which lacks sufficient Fe²⁺ ions needed to react with cyanide ions to form hexacyanoferrate(II).

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