NEET Practice Questions (MCQs) with Answers & Solutions

Kjeldahl’s method cannot be used for the estimation of nitrogen in DNA and RNA because these molecules contain nitrogen in heterocyclic rings, which do not break down under the conditions of the Kjeldahl method.

Iodine can be separated from sodium chloride using the method of sublimation. Sublimation is the process where a solid changes directly to a gas without passing through the liquid state. Iodine is a substance that can sublime, while sodium chloride does not. Hence, when the mixture is heated, iodine will turn into vapor and can be collected after cooling, leaving behind sodium chloride.

The formula for iron (III) hexacyanoferrate (II) is $ Fe_4[Fe(CN)_6]_3 $. In this compound, iron exists in two oxidation states: Fe(III) and Fe(II). The hexacyanoferrate (II) ion is $ [Fe(CN)_6]^{4-} $, and when combined with Fe(III) ions, the overall formula becomes $ Fe_4[Fe(CN)_6]_3 $.

Fractional distillation is used to separate mixtures of liquids with different boiling points. Since the boiling points of liquids A and B differ by 15K, fractional distillation is the appropriate method for their separation. This method utilizes a fractionating column to separate the components based on their boiling points.

Solvent extraction is used to separate an organic compound from its aqueous solution. This process involves transferring a solute from one solvent into another based on the differing solubilities of the solute in the two solvents, making it an effective method for separation.

$ let the vol of acid left unused = V ml of 0.1 MH_2SO_4 $ $ \therefore V \times 0.1 \times 2 = 20 \times 0.5\times 1 $ $ V = 50 ml = 50 ml of .01 m H_2SO_4 $ $ Vol of acid used = 100 - 50 $ $ % N = { 1.4 \times 2 \times 50\times 0.1 \over 0.3 } = 46.6 $ $ %N in urea = { 28 \over 60} \times 100 = 46.6 $

Thiourea gives a blood red coloration when its Lassaigne extract is treated with alkali and ferric chloride. This is due to the formation of a complex between the thiourea and ferric ions, which results in the characteristic blood red color.

Lassaigne test for nitrogen involves the fusion of the organic compound with sodium, which converts nitrogen present in the compound into sodium cyanide ($NaCN$). Only $NaCN$ among the given options will form sodium cyanide, which can be detected by the formation of Prussian blue color when treated with iron(II) sulfate and then acidified.

In the Lassaigne test, p-aminobenzene sulphonic acid will give a blood red color due to the formation of ferric thiocyanate. This happens because the sulfur in the compound reacts with sodium to form sodium sulfide, which then reacts with iron(III) to form ferric thiocyanate, giving a blood red color.