All questions of Purification and Characterisation of Organic Compounds for NEET Exam

The best method for the separation of naphthalene and benzoic acid from their mixture is sublimation because it is applicable for those organic compounds which pass directly from solid to vapour state on heating and vice versa on cooling.
In these compounds naphthalene is volatile and benzoic acid is non-volatile due to the formation of dimer via hydrogen bonding (intermolecular).

Carboxylic acids are soluble in sodium bicarbonate but phenol are not dissolve in it, so they are separated because carboxylic acid react with NaHCO₃ and form sodium carboxylate.
R — COOH + NaHCO₃ → R — COO⁻Na⁺ + H₂CO₃

Camphor is used in molecular mass determination due to its volatile nature.
The method is called Rast’s camphor method. Camphor acts as a solid solvent which is volatile, hence can be removed easily.

In steam distillation of toluene, the pressure of toluene in vapour is less than pressure of barometer, because it is carried out when a solid or liquid is insoluble in water and is volatile with steam but the impurities are non-volatile.

Being hygroscopic, sodium dichromate, Na₂Cr₂O₇ cannot be used in volumetric analysis.
All other given statements are true.

Steam distillation is used to purify the substances which
(i) are volatile in steam but are immiscible with water.
(ii) possess sufficiently high vapour pressure at the boiling point of water.
(iii) contain non- volatile impurities.
The process of steam distillation can also be used to separate a mixture of two organic compounds one of which is steam volatile while the other is not.
In ortho and para-nitrophenol, ortho-nitrophenol has intramolecular H-bonding. So, it has lower boiling point.
Intermolecular H-Bonding more strong then intramolecular H-bonding. Whereas para-nitrophenol has intermolecular H-bonding. So, it has higher boiling point.
Due to difference in boiling points ortho and para-nitrophenol can be separated from each other by distillation.

IR spectroscopy is used for the purification of cyclohexanone from a mixture of benzoic acid, isoamyl alcohol, cyclohexane and cyclohexanone because in IR spectroscopy each functional group appears at a certain peak. IR spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure.

When the sodium fusion extract is added with FeCl₃ and then the resulting solution is acidified with dilute hydrochloric acid, the appearance of Prussian blue colouration confirms the presence of nitrogen in the organic compound.

Lassaigne’s test is given by only those compounds which contain both carbon and nitrogen. When compounds containing C and N heated with sodium, then it form NaCN which is easily detected byFeCl₃.
Or
Some compounds live hydrazine (NH₂⋅ NH₂) although contain nitrogen but they do not respond Lassaigne’s test because they do not have any carbon and hence, NaCN is not formed.

Na₂S and NaCN, if present in the extract, will be decomposed to H₂S and HCN by HNO₃. 
NaCN + HNO₃ → NaNO₃ + HCN
Na₂S + 2HNO₃ → 2NaNO₃ + H₂S
These will escape from the solution and will not interfere with the test for halogens.