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Detection of nitrogen, sulphur and halogens in organic compounds: NEET MCQs with answers

50 questions on this topic of Principles Related to Practical Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Given below are two statements: Statement I: In the sodium nitroprusside test for sulphur, the reaction takes place under alkaline conditions. Statement II: Sodium nitroprusside has the chemical formula Na₂[Fe(CN)₅NO]. In light of the above statements, choose the correct answer:

  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
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Statement I is correct because sodium fusion extract is inherently alkaline, and the reaction S²⁻ + [Fe(CN)₅NO]²⁻ → [Fe(CN)₅NOS]⁴⁻ occurs smoothly in alkaline medium. Statement II is also correct; sodium nitroprusside is sodium pentacyanonitrosylferrate(II), Na₂[Fe(CN)₅NO].

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2. During the detection of nitrogen by the Prussian blue test, what is the role of adding acidified FeCl₃ or allowing the aerial oxidation of Fe²⁺?

  1. It precipitates iron as insoluble iron(III) hydroxide
  2. It reduces unreacted NaCN to form elemental nitrogen gas
  3. It neutralizes the excess sodium hydroxide without changing iron oxidation state
  4. It provides Fe³⁺ ions required to react with hexacyanidoferrate(II) to form Prussian blue
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Answer: (D) It provides Fe³⁺ ions required to react with hexacyanidoferrate(II) to form Prussian blue

In the Prussian blue test, Fe²⁺ reacts with CN⁻ to form [Fe(CN)₆]⁴⁻. For Prussian blue (Fe₄[Fe(CN)₆]₃) to precipitate, Fe³⁺ ions are required, which are either supplied by aerial oxidation of Fe²⁺ during boiling or by deliberately adding FeCl₃.

hard

3. Consider the following statements regarding tests for sulphur in Lassaigne's extract: Statement I: Acidification with dilute sulphuric acid instead of acetic acid before adding lead acetate can lead to a false positive white precipitate of lead sulphate. Statement II: The sodium nitroprusside test must be performed in a strongly acidic medium to stabilize the violet complex. Choose the correct option:

  1. Statement I is correct, but Statement II is incorrect
  2. Statement I is incorrect, but Statement II is correct
  3. Both Statement I and Statement II are correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Statement I is correct, but Statement II is incorrect

Statement I is correct because H₂SO₄ yields SO₄²⁻ ions which precipitate with Pb²⁺ as white PbSO₄, obscuring or mimicking the test. Statement II is incorrect because the sodium nitroprusside test requires an alkaline medium for the formation of the purple [Fe(CN)₅NOS]⁴⁻ complex.

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4. An organic compound containing carbon, hydrogen, nitrogen, and sulphur was subjected to sodium fusion using a slight deficiency of sodium metal. The resulting extract, upon reaction with neutral FeCl₃, gave a blood-red coloration. What is the formula of the complex ion responsible for this coloration?

  1. [Fe(CN)₆]³⁻
  2. [Fe(SCN)]²⁺
  3. [Fe(SCN)₆]⁴⁻
  4. [Fe(H₂O)₄(SCN)₂]⁺
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Answer: (B) [Fe(SCN)]²⁺

When insufficient sodium is used with a compound containing both N and S, sodium thiocyanate (NaSCN) forms instead of separate NaCN and Na₂S. Reaction with Fe³⁺ produces the blood-red thiocyanatoiron(III) complex, predominantly [Fe(SCN)]²⁺ or [Fe(SCN)(H₂O)₅]²⁺.

easy

5. Given below are two statements: Statement I: Hydrazine sulphate (NH₂NH₂ · H₂SO₄) fails to give Prussian blue coloration in Lassaigne's test for nitrogen. Statement II: In Lassaigne's test, the conversion of nitrogen to sodium cyanide requires the presence of carbon in the organic compound. In light of the above statements, choose the most appropriate answer:

  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.
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct.

Hydrazine sulphate contains nitrogen and sulphur but lacks carbon. During sodium fusion, nitrogen cannot be converted into sodium cyanide (NaCN) without carbon, so no Prussian blue is formed. Both statements are correct, and Statement II explains why hydrazine fails the test.

hard

6. In the layer test for halogens, an acidified sodium fusion extract is shaken with CHCl₃ and chlorine water. The organic layer turns orange-brown. Upon adding excess chlorine water with vigorous shaking, the colour remains orange-brown. This confirms the presence of:

  1. Iodine
  2. Bromine
  3. Chlorine
  4. Fluorine
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Answer: (B) Bromine

Chlorine water oxidizes Br⁻ to Br₂, which dissolves in CHCl₃ giving an orange-brown layer that remains unchanged with excess chlorine water. In contrast, I₂ forms a violet layer that becomes colourless with excess chlorine water due to oxidation to iodic acid (HIO₃).

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7. In the test for chlorine, a white precipitate with AgNO₃ that is soluble in ammonium hydroxide indicates:

  1. Iodide (AgI)
  2. Sulphide
  3. Cyanide
  4. Chloride (AgCl)
Show answer and explanation

Answer: (D) Chloride (AgCl)

AgCl is white and readily soluble in NH₄OH. AgBr is pale yellow and sparingly soluble, and AgI is yellow and insoluble.

NCERT Class 11

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8. Consider the following statements regarding the detection of nitrogen by Lassaigne's test: Statement I: Diazonium salts like C₆H₅N₂⁺Cl⁻ fail to give a positive Lassaigne's test for nitrogen because they decompose and lose nitrogen gas before fusion can occur. Statement II: Sodium fusion extract is always alkaline because of the reaction of unreacted sodium metal with water during extraction. Choose the correct option:

  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
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Diazonium salts lose N₂ gas readily on mild heating before sodium can reduce and fuse with them to form NaCN; thus they do not give a positive Lassaigne's test. Additionally, plunging the red-hot tube into distilled water causes excess unreacted sodium to react violently with water forming NaOH, making the extract alkaline. Both statements are correct.

easy

9. What is the stoichiometric ratio of iron in +3 and +2 oxidation states respectively in the chemical formula of Prussian blue, Fe₄[Fe(CN)₆]₃?

  1. 4 : 3
  2. 3 : 4
  3. 1 : 1
  4. 2 : 3
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Answer: (A) 4 : 3

In the chemical formula of Prussian blue, Fe₄[Fe(CN)₆]₃ · xH₂O, there are four external ferric ions (Fe³⁺) and three hexacyanoferrate(II) units containing ferrous ions (Fe²⁺). Thus, the ratio of Fe³⁺ to Fe²⁺ is 4 : 3.

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10. In the sodium nitroprusside test for sulphur, the appearance of a violet or purple coloration is due to the formation of:

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

Sodium sulphide present in Lassaigne's extract reacts with sodium nitroprusside according to the equation: Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS]. The resulting sodium thionitroprusside complex contains the anion [Fe(CN)₅NOS]⁴⁻.

easy

11. A Lassaigne's extract of an organic compound gives a pale yellow precipitate upon treatment with dilute HNO₃ and AgNO₃ solution. The precipitate is sparingly soluble in dilute aqueous NH₄OH but completely dissolves in concentrated NH₄OH. The halogen present in the organic compound is:

  1. Chlorine
  2. Fluorine
  3. Iodine
  4. Bromine
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Answer: (D) Bromine

Silver bromide (AgBr) forms as a pale yellow precipitate that is sparingly soluble in dilute aqueous ammonia but dissolves in concentrated ammonium hydroxide solution. In contrast, AgCl is a curdy white precipitate readily soluble in dilute ammonia, and AgI is bright yellow and insoluble even in concentrated ammonia.

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12. What is the primary role of adding freshly prepared FeSO₄ rather than an old, stored sample of FeSO₄ during the Lassaigne's test for nitrogen?

  1. Old FeSO₄ solutions get oxidised by air to basic ferric sulphate, lacking sufficient Fe²⁺ ions to form hexacyanidoferrate(II)
  2. Freshly prepared FeSO₄ generates nascent hydrogen required for cyanide reduction
  3. Stored FeSO₄ solution decomposes completely into sulphur dioxide gas and iron metal
  4. Stored FeSO₄ contains excess dissolved oxygen which reacts vigorously with sodium cyanide
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Answer: (A) Old FeSO₄ solutions get oxidised by air to basic ferric sulphate, lacking sufficient Fe²⁺ ions to form hexacyanidoferrate(II)

A stored solution of ferrous sulphate undergoes atmospheric oxidation to form basic ferric salts and ferric sulphate, resulting in an inadequate concentration of Fe²⁺ ions needed to react with cyanide ions to form sodium hexacyanidoferrate(II). Therefore, a freshly prepared FeSO₄ solution must be used.

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