Chemistry MCQs for NEET — Practice Questions with Answers

The basic principle of the Lassaigne test is to convert the elements present in the organic compound from their covalent form to their ionic form. This conversion allows for easier detection of elements such as sulfur, nitrogen, and halogens.

The Prussian blue color obtained in the Lassaigne test is due to the formation of $Fe_4[Fe(CN)_6]$. This compound is formed when the ionic cyanide formed during the test reacts with ferrous sulfate, producing the characteristic Prussian blue color.

In the Lassaigne test for the detection of Sulphur, a black precipitate is formed due to the formation of lead sulfide (PbS). When the sodium fusion extract (Lassaigne's extract) containing sulfur is treated with lead acetate, a black precipitate of lead sulfide is obtained. The reaction can be represented as: \[ Na_2S + (CH_3COO)_2Pb \rightarrow PbS + 2CH_3COONa \\]

When a Lassaigne solution containing sulfur is treated with sodium nitroprusside, a violet color is observed. This is because sulfur present in the solution reacts with sodium nitroprusside to form a violet-colored complex. The reaction can be represented as: \[ Na_2S + Na_2[Fe(CN)_5NO] \rightarrow Na_4[Fe(CN)_5NOS] \\]

The violet color observed in the Lassaigne test for sulfur is due to the formation of the complex ion \[ [Fe(CN)_5NOS]^{4-} \\]. This complex is formed when the sulfur in the sample reacts with sodium nitroprusside in the solution. The reaction can be represented as: \[ Na_2S + Na_2[Fe(CN)_5NO] \rightarrow Na_4[Fe(CN)_5NOS] \\]

During the estimation of Carbon (C) and Hydrogen (H), the carbon dioxide ($CO_2$) produced is absorbed in potassium hydroxide (KOH). This is because KOH reacts with $CO_2$ to form potassium carbonate ($K_2CO_3$), effectively removing $CO_2$ from the mixture.

During the estimation of Carbon (C) and Hydrogen (H), the water ($H_2O$) produced is absorbed in anhydrous calcium chloride ($An.CaCl_2$). Anhydrous calcium chloride is a highly hygroscopic substance, meaning it readily absorbs moisture from its surroundings.

In Kjeldahl’s method for the estimation of nitrogen, the formula used is $ \\% N = { 1.4 NV \over W} $. Here, N is normality of acid, V is volume of acid, and W is the weight of the sample. This formula helps in calculating the percentage of nitrogen in the given sample.