Question

1. A student wanted to crystallize P-Toluic acid (structure and melting point shown below) and therefore obtained the solubility information for crystallization of this compound from literature with four solvents, A, B, C, and D presented below in the table.   

Solvent	Boiling point (oC)	Solubility in g/mL at room temperature	Solubility in g/mL at bp
A	220	0.13 g	43 g
B	80	4.8 g	12 g
C	172	0.24 g	36 g
D	168	2.4 g	28 g

1. Which solvent would you recommend the student for carrying out the crystallization of P-Toluic acid?

2. Give three reasons for recommending that solvent.

Answer 1

a. The solvent B is recommended for carrying out the crystallization of p-toluic acid.

b. The reasons for recommending that solvent are as follows:

• For crystallization of the solute, it must form a saturated solution first with the solvent so that when excess solute is added, the doesn't dissolve further and it starts forming crystals. The best saturated solution should have the minimum solubility at boiling point.
• The boiling point of the solvent B is comparatively lower than other solvents which means it can boil faster.
• The amount of solute to be added to the solvent from room temperature to boiling point is least (12 - 4.3 = 7.7) for solvent B as compared to other solvents. So less amount of solute will be retained in the solution and excess solute will get crystallized.