Impure benzoic acid was dissolved in hot water. The container of solution was pl
ID: 1006976 • Letter: I
Question
Impure benzoic acid was dissolved in hot water. The container of solution was placed in an instead of being allowed cooling slowly. What will be the result of cooling the solution in this manner? Outline the successive steps in the crystallization of an organic solid from a solvent and state the purpose of each operation. Compound X is quite soluble in toluene, but only slightly soluble in petroleum ether. How could these solvents be used in combination in order to recrystallize X? 0.12 g of compound "Y" dissolves in 10 ml of acetone at 25 degree C and 0.85 g of the same compound dissolves in 10 ml of boiling acetone. What volume of acetone would be required to purify a 5.0 g sample of compound?Explanation / Answer
1) Dissolve solid product in minimum volume of hot solvent .Purpose ... to get into solution
2) cool solution in a mixture of ice, water . Purpose .. to enable pure product to crystallise,as solubility decreases with temperature. Impurities are present in small quantities, so do not saturate and are left in solution.
3) If it all sets solid, you haven't used enough solvent, so add a little more and warm until everything dissolves. Purpose: you need solid and supernatent liquid to effectively filter and remove impurities.
4) Filter preferably by Buchner filtration. Purpose to remove surplus solvent.
5) Wash filtrate with a little cold solvent. Purpose .. solid will be wet with solvent containing impurites .. need to wash this away .. but not so much that you lose a lot of your yield by it redissolving in solvent
6) let suction continue for a while. Purpose to remove bulk of washing solvent
7) remove filter pad, and scrape product onto a pad of several bits of dry filter paper and spread it out using a spatula. Purpose: to let last bit of solvent evaporate to give pure, dry product. This is vital or melting point will be inaccurate.
8) for normal lab, where both percentage yield and melting point are important, this will do. However if you want a very pure product or have to check the melting point, repeat the whole process. Purpose: to get very pure product and 'spot on' melting point. You will however get a slightly lower yield.