Question

A student weighted out 100mg sample semi-pure sample of salicylic acid. After recrystallization from ethyl acetate, the yield was was 22mg. (A) Where is the remaining compound, (B) why is it there, and (C) how can the yield be improved? (the answer is not simply concentrate the filtrate and recrystallize from ethyl acetate again) Answer A, B, and C in complete sentences.

Answer 1

A student weighted out 100mg sample semi-pure sample of salicylic acid. After recrystallization from ethyl acetate, the yield was was 22mg. (A) Where is the remaining compound,

Ans: The remaining compound is present in the mother liquor (filtrate). Also some amount of salicylic acid is stuck to the filter paper.

(B) why is it there,

Ans: If more than required amount of ethyl acetate was used during the crystallisation, some salicylic acid will remain dissolved in the excess ethyl acetate. Also if during the crystallisation, the flask is not properly cooled, the temperature of flask may be higher than required which will result in higher solubility of salicylic acid in ethyl acetate.

(C) how can the yield be improved?

Ans: To improve the yield, it is necessary that the mother liquor should be made super saturated solution so that no more salicylic acid can dissolve in it and the salicylic acid that is dissolved in it should be precipitated. To achieve this, we need to increase the concentration of salicylic acid in ethyl acetate by reducing the amount of ethyl acetate present for a given amount of salicylic acid. We concentrate the mother liquor by heating so that some ethyl acetate is evoparated. Now the amount of salicylic acid present per ml of ethyl acetate is more than that present before concentration. Now we again crystallise the solution by colling. This time we ensure that the flask is properly cooled. This time we will get better yield.