jee main chemistry important questions | Purification And Characterization Of Organic Compounds

Q 1 :

The Lassaigne’s extract is boiled with dilute ${HNO}_{3}$ , before testing for halogens because, [2024]

AgCN is soluble in ${HNO}_{3}$ .
Silver halides are soluble in ${HNO}_{3}$ .
${Ag}_{2} S$ is soluble in ${HNO}_{3}$ .
${Na}_{2} S$ and NaCN are decomposed by ${HNO}_{3}$ .

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(4)

If the salt contains sulphide or cyanide, then Lassaigne’s extract would contain ${Na}_{2} S$ or NaCN. In such cases, upon the addition of ${AgNO}_{3}$ in the extract, ${Ag}_{2} S$ (black precipitate) or AgCN (white residue) are obtained which interferes with the test for halides. Hence Lassaigne’s extract is boiled with dil. ${HNO}_{3}$ to decompose ${Na}_{2} S$ or NaCN.

Q 2 :

Lassaigne’s test is used for detection of: [2024]

Phosphorous and halogens only
Nitrogen, Sulphur and Phosphorous only
Nitrogen, Sulphur, Phosphorous and halogens
Nitrogen and Sulphur only

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(3)

Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s test”.

Q 3 :

Which of the following nitrogen containing compound does not give Lassaigne’s test? [2024]

Glycine
Hydrazine
Phenyl hydrazine
Urea

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(2)

In Lassaigne's test for nitrogen, NaCN is formed in sodium fusion extract, which is then tested further. Hydrazine ( ${NH}_{2} - {NH}_{2}$ ) does not contain carbon hence NaCN cannot be formed in sodium fusion extract. So hydrazine does not give Lassaigne 's test for nitrogen.

Q 4 :

Given below are two statements: [2025]

Statement I: Nitrogen, sulphur, halogen and phosphorus present in an organic compound are detected by Lassaigne’s Test.

Statement II: The elements present in the compound are converted from covalent form into ionic form by fusing the compound with Magnesium in Lassaigne’s test.

In the light of the above statements, choose the correct answer from the options given below:

Statement I is false but Statement II is true
Both Statement I and Statement II are false
Both Statement I and Statement II are true
Statement I is true but Statement II is false

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(4)

Elements are converted from covalent to ionic form by fusion with sodium metal.

Q 5 :

Given below are two statements: [2025]

Statement I: In Lassaigne’s test, the covalent organic molecules are transformed into ionic compounds.

Statement II: The sodium fusion extract of an organic compound having N and S gives prussian blue colour with ${FeSO}_{4}$ and ${Na}_{4} [Fe {(CN)}_{6}]$ .

In the light of the above statements, choose the correct answer from the options given below:

Statement I is false but Statement II is true
Both Statement I and Statement II are false
Both Statement I and Statement II are true
Statement I is true but Statement II is false

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(4)

If an organic compound has both S and N, then its sodium fusion extract gives blood red colour during test of N.

Q 6 :

Compound that will give positive Lassaigne’s test for both nitrogen and halogen is: [2023]

${NH}_{2} OH . HCl$
${CH}_{3} {NH}_{2} . HCl$
${NH}_{4} Cl$
$N_{2} H_{4} . HCl$

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(2)

Lassaigne test for both nitrogen and halogen is given by the compound which have C, N as well as X atom in the compound.

Q 7 :

Which of the following compounds would give the following set of qualitative analysis? [2023]

(i) Fehling’s Test: Positive

(ii) Na fusion extract upon treatment with sodium nitroprusside gives a blood-red colour but not Prussian blue.

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[IMAGE 64]
[IMAGE 65]
[IMAGE 66]

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(2)

Fehling test given by aldehyde but not aromatic aldehyde and if N and S attached with carbon atom give blood red with sodium nitroprusside gives a blood red colour but not prussian blue.

Q 8 :

Given below are two statements:

Statement I: Sulphanilic acid gives esterification test for carboxyl group.

Statement II: Sulphanilic acid gives red colour in Lassaigne’s test for extra element detection.

In the light of the above statements, choose the correct answer from the options given below: [2023]

Both Statement I and Statement II are correct
Both Statement I and Statement II are incorrect
Statement I is incorrect but Statement II is correct
Statement I is correct but Statement II is incorrect

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(3)

Sulphanilic acid has no –COOH group hence no esterification test for carboxyl group. But it has sulpher and nitrogen atom hence gives red colour in Lassigne’s test for extra element detection.

Q 9 :

Match List I with List II. [2023]

List I List II

Element Detected Reagent used/Product formed

A. Nitrogen I. ${Na}_{2} [Fe {(CN)}_{5} NO]$

B. Sulphur II. ${AgNO}_{3}$

C. Phosphorus III. ${Fe}_{4} {[Fe {(CN)}_{6}]}_{3}$

D. Halogen IV. ${({NH}_{4})}_{2} {MoO}_{4}$

Choose the correct answer from the options given below:

A–III, B–I, C–IV, D–II
A–II, B–IV, C–I, D–III
A–II, B–I, C–IV, D–III
A–IV, B–II, C–I, D–III

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(1)

(A) $‘N’ + {[Fe {(CN)}_{6}]}^{4 -} \to Blue colour$

(B) $‘S’ + Sodium nitroprusside \to Violet colour$

(C) $‘P’ + {({NH}_{4})}_{2} {MoO}_{4} \to {({NH}_{4})}_{3} {PO}_{4} . 12 {MoO}_{3} \to Yellow colour$

(D) $Halogen + {AgNO}_{3} \to AgX ppt$

Topic Question Set

Q 1 :

The Lassaigne’s extract is boiled with dilute ${HNO}_{3}$ , before testing for halogens because, [2024]

Q 2 :

Lassaigne’s test is used for detection of: [2024]

Q 3 :

Which of the following nitrogen containing compound does not give Lassaigne’s test? [2024]

Q 6 :

Compound that will give positive Lassaigne’s test for both nitrogen and halogen is: [2023]

Q 7 :

Which of the following compounds would give the following set of qualitative analysis? [2023]

(i) Fehling’s Test: Positive

(ii) Na fusion extract upon treatment with sodium nitroprusside gives a blood-red colour but not Prussian blue.

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	List I		List II
	Element Detected		Reagent used/Product formed
A.	Nitrogen	I.	${Na}_{2} [Fe {(CN)}_{5} NO]$
B.	Sulphur	II.	${AgNO}_{3}$
C.	Phosphorus	III.	${Fe}_{4} {[Fe {(CN)}_{6}]}_{3}$
D.	Halogen	IV.	${({NH}_{4})}_{2} {MoO}_{4}$

Topic Question Set

Q 2 : Lassaigne’s test is used for detection of: [2024] .question-row { display: flex; align-items: flex-start; gap: 8px; } .q-no { font-size: 17px; white-space: nowrap; margin-left: -15px; } .q-text { font-size: 18px; flex: 1; } .q-no, .q-text p { line-height: 30px; }

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Q 2 :

Lassaigne’s test is used for detection of: [2024]