1st year chemistry - 2nd chapter exercise short Question

Q.4. Why is there a need to crystallize the crude product?

Ans: When a compound is prepared in laboratory, it may contain impurities. This impure and un-refined compound is called crude product. It is necessary to purify the crude product. So there is a need to crystallize the crude product.

Q.5. A water insoluble organic compound aspirin is prepared by the reaction of salicylic acid with a mixture of acetic acid and acetic anhydride. How will you separate the product from the reaction mixture?

Ans: Aspirin is separated from reaction mixture is added to cold water. The aspirin forms crystals and other products remain in solution. Finally aspirin is filtered from water by sintered glass crucible.

Q.6. A solid organic compound is soluble in water as well as in chloroform. During its preparation, it remains in aqueous layer. Describe a method to obtain it from this layer.

Ans: The organic compound can be extracted by solvent extraction. The aqueous solution of compound is mixed with carbon tetrachloride (CCl4). The mixture is put into the separating funnel. Here two layers are formed. The water layer is separated and evaporated to get the compound.

Q.7. The following figure shows a developed chromatogram on paper with five spots. (i) Unknown mixture X (ii) Sample A (iii) Sample B (iv) Sample C (v) Sample D

Find out (i) the composition of mixture X (ii) which sample is impure & what is its composition.

Ans: The unknown mixture X contains the components B and C. The sample D is impure. It contains components A and C. We should know that a pure sample give only one spot. The samples A and C are pure.

Q.8. In solvent extraction technique, why repeated extraction using small portions of solvent are more efficient than using a single extraction but larger volume of solvent?

Ans: The solvent extraction technique is based upon distribution coefficient. The ratio of concentration of a solute dissolved in two immiscible solvents is a constant. It is called distribution coefficient.

In repeated extractions, we can get maximum amount of solute from the other solvent. It is the reason that repeated extractions using small portions of solvent are more efficient than using a single extraction but larger volume of solvent.