Chemistry Class 11 NCERT Solutions: Chapter 12 Organic Chemistry Some Basic Principles and Techniques Part 14

Q: 24. Explain the principle of paper chromatography.

Answer:

In paper chromatography, chromatography paper is used. This paper contains water trapped in it, which acts as the stationary phase. On the base of this chromatography paper, the solution of the mixture is spotted. The paper strip is then suspended in a suitable solvent, which acts as the mobile phase. This solvent rises up the chromatography paper by capillary action and in the procedure, it flows over the spot. The components are selectively retained on the paper (according to their differing partition in these two phases). The spots of different components travel with the mobile phase to different heights. The paper so obtained (shown in the given figure) is known as a chromatogram.

Chromatography paper in solvent

Principle of Chromatography Paper

Chromatography paper in solvent

Q: 25. Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?

Answer:

While testing the Lassaigne’s extract for the presence of halogens, it is first boiled with dilute nitric acid. This is done to decompose Equation to Equation and Equation to Equation and to expel these gases. That is, if any nitrogen and sulphur are present in the form of Equation and Equation , then they are removed. The chemical equations involved in the reaction are represented as

Equation

Equation

Q: 26. Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.

Answer:

Nitrogen, sulphur, and halogens are covalently bonded in organic compounds. For their detection, they have to be first converted to ionic form. This is done by fusing the organic compound with sodium metal. This is called “Lassaigne’s test”. The chemical equations involved in the test are

Equation

Equation

Equation

Equation

Carbon, nitrogen, sulphur, and halogen come from organic compounds.

Q: 27. Name a suitable technique of separation of the components from a mixture of calcium sulphate and camphor.

Answer:

The process of sublimation is used to separate a mixture of camphor and calcium sulphate. In this process, the sublimable compound changes from solid to vapour state without passing through the liquid state. Camphor is a sublimable compound and calcium sulphate is a non3sublimable solid. Hence, on heating, camphor will sublime while calcium sulphate will be left behind.

Q: 28. Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation?

Answer:

In steam distillation, the organic liquid starts to boil when the sum of vapour pressure due to the organic liquid Equation and the vapour pressure due to water Equation becomes equal to atmospheric pressure Equation , that is, Equation

Since Equation , organic liquid will vapourise at a lower temperature than its boiling point.

Q: 29. Will Equation give white precipitate of Equation on heating it with silver nitrate? Give reason for your answer.

Answer:

Equation will not give the white precipitate of Equation on heating it with silver nitrate. This is because the chlorine atoms are covalently bonded to carbon in Equation . To obtain the precipitate, it should be present in ionic form and for this, it is necessary to prepare the Lassaigne’s extract of Equation .

Q: 30. Why is a solution of potassium hydroxide used to absorb carbon dioxide evolved during the estimation of carbon present in an organic compound?

Answer:

Carbon dioxide is acidic in nature and potassium hydroxide is a strong base. Hence, carbon dioxide reacts with potassium hydroxide to form potassium carbonate and water as

Equation

Thus, the mass of the U - tube containing Equation increases. This increase in the mass of U- tube gives the mass of Equation produced. From its mass, the percentage of carbon in the organic compound can be estimated.

Q: 31. Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?

Answer:

Although the addition of sulphuric acid will precipitate lead sulphate, the addition of acetic acid will ensure a complete precipitation of sulphur in the form of lead sulphate due to common ion effect. Hence, it is necessary to use acetic acid for acidification of sodium extract for testing sulphur by lead acetate test.

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