Chemistry Class 11 NCERT Solutions: Chapter 12 Organic Chemistry Some Basic Principles and Techniques Part 13

Q: 22. Differentiate between the principle of estimation of nitrogen in an organic compound by

(I) Dumas method and

(II) Kjeldahl’s method

Answer:

In Dumas method, a known quantity of nitrogen containing organic compound is heated strongly with excess of copper oxide in an atmosphere of carbon dioxide to produce free nitrogen in addition to carbon dioxide and water. The chemical equation involved in the process can be represented as

Equation

The traces of nitrogen oxides can also be produced in the reaction, which can be reduced to dinitrogen by passing the gaseous mixture over a heated copper gauge. The dinitrogen produced is collected over an aqueous solution of potassium hydroxide. The volume of nitrogen produced is then measured at room temperature and atmospheric pressure.

On the other hand, in Kjeldahl’s method, a known quantity of nitrogen containing organic compound is heated with concentrated sulphuric acid. The nitrogen present in the compound is quantitatively converted into ammonium sulphate. It is then distilled with excess of sodium hydroxide. The ammonia evolved during this process is passed into a known volume of H2SO4. The chemical equations involved in the process are

The acid that is left unused is estimated by volumetric analysis (titrating it against a standard alkali) and the amount of ammonia produced can be determined. Thus, the percentage of nitrogen in the compound can be estimated. This method cannot be applied to the compounds, in which nitrogen is present in a ring structure, and also not applicable to compounds containing nitro and Equation groups.

Equation

Equation

Equation

Q: 23. Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.

Answer:

Estimation of Halogens

Halogens are estimated by the Carius method. In this method, a known quantity of organic compound is heated with fuming nitric acid in the presence of silver nitrate, contained in a hard glass tube called the Carius tube, taken in a furnace. Carbon and hydrogen that are present in the compound are oxidized to form CO2 and H2O respectively and the halogen present in the compound is converted to the form of Equation . This Equation is then filtered, washed, dried, and weighed.

Let the mass of organic compound be Equation .

Mass of Equation formed Equation

Equation mol of Equation contains 1 mol of X.

Therefore,

Mass of Halogen in Equation

Equation

Estimation of Sulphur

In this method, a known quantity of organic compound is heated with either fuming nitric acid or sodium peroxide in a hard glass tube called the Carius tube. Sulphur, present in the compound, is oxidized to form sulphuric acid. On addition of excess of barium chloride to it, the precipitation of barium sulphate takes place. This precipitate is then filtered, washed, dried, and weighed.

Let the mass of organic compound be Equation .

Mass of Equation formed Equation

Equation mol of Equation of Sulphur

Therefore, Equation

Equation

Thus, percentage of sulphur Equation

Estimation of phosphorus

In this method, a known quantity of organic compound is heated with fuming nitric acid. Phosphorus, present in the compound, is oxidized to form phosphoric acid. By adding ammonia and ammonium moly date to the solution, phosphorus can be precipitated as ammonium phosphomolybdate.

Phosphorus can also be estimated by precipitating it as Equation by adding magnesia mixture, which on ignition yields Equation .

Let the mass of organic compound be m. g.

Mass of ammonium phosphomolybdate formed Equation

Molar mass of ammonium phosphomolybdate Equation

Thus, percentage of phosphorus Equation

If P is estimated as Equation

Then, percentage of phosphorus Equation

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