Chemistry MCQs for NEET — Practice Questions with Answers

In Kjeldahl’s method, the nitrogenous organic compound is heated with concentrated sulfuric acid. During this process, nitrogen is converted into ammonium sulfate, $(NH_4)_2SO_4$. The reaction can be represented as: \[ \text{Organic compound} + H_2SO_4 \rightarrow (NH_4)_2SO_4 \] This conversion is crucial for the subsequent determination of nitrogen content.

$ 1M of 10 ml H_2SO_4 = 1 M of 20ml NH_3 $ $ 1000ml of 1M NH_3 = 14 g nitrogen $ $ 20 ml of 1M NH_3 ={14 \times 20 \over 1000} g nitrogen $ $ %N = { 14 \times 20 \times 100 \over 1000 \times 0.5 } = 56.0% $

In the Carius method for halogen estimation, the halogen present in the organic compound is converted into silver halide (AgX) by reacting with silver nitrate (AgNO_3). The reaction is as follows: \[ \text{R-X} + AgNO_3 \rightarrow AgX + \text{R-NO_3} \] where R-X is the organic compound containing the halogen. Silver halide precipitates out and can be weighed to determine the halogen content.

In the estimation of sulfur in an organic compound, the sulfur is typically converted to sulfate ion (SO₄²⁻), which is then precipitated as barium sulfate (BaSO₄). The amount of sulfur is determined by measuring the mass of BaSO₄ formed. So, the correct answer is BaSO₄.

In the estimation of phosphorus in organic compounds, the phosphorus is precipitated as magnesium ammonium phosphate. Upon ignition, it is converted to magnesium pyrophosphate (Mg2P2O7), which is weighed to determine the amount of phosphorus. This method ensures accurate quantification of phosphorus content.

Sublimation is the best method to purify camphor from common salt. Camphor sublimes, i.e., it transitions directly from solid to vapor without passing through a liquid phase, while common salt does not. This property allows camphor to be separated effectively from the non-sublimable impurities like common salt.

Fractional distillation is used to separate a mixture of methanol and acetone. Both substances have different boiling points (methanol boils at 64.7°C and acetone at 56.0°C). Fractional distillation allows for the separation of components in a mixture based on differences in boiling points, making it suitable for separating methanol and acetone.

Sublimation is the method used to purify naphthalene because naphthalene sublimes, which means it can transition directly from a solid to a gas without passing through a liquid state. This property allows for its purification by heating the impure sample so that the naphthalene vaporizes and then condenses back into a solid upon cooling.