Even if two compounds have identical solubility profiles in a solvent, it is pos

Question

Even if two compounds have identical solubility profiles in a solvent, it is possible to effect some purification by a single recrystallization. Assume you have a 10 g mixture of A and B in a 9:1 ratio of A to B and that both have a solubility at room temperature of 0.08 g/mL. If you used 20 mL of solvent for recrystallization, how much B would be expected to be in the crystallized material? Hint: calculate how much of A and how much of B would be expected to remain dissolved in the solvent after cooling to room temperature. (This calculation assumes an ideal recrystallization that probably wouldn

Explanation / Answer

If we assume that the solvent will hold 0.08g/ml of A and B (i.e 0.064 g/ml of each) at room temperature and that the whole of the mixture will dissolve in the boiling solvent then we can calculate that (at room temperature) there will remain in solution 20 x 0.064g = 1.28 g of each solid. This leaves a mixture of 7.72 g of A and 1.28 g of B in the substrate after filtration. Ratio A:B = 15.44 : 2.64 If the solubility of each substance is unaffected by the presence of the other then 20 ml of solvent will dissolve 1 g of A and 1g of B at room temp. leaving 8g of pure A undissolved which may be filtered off and washed to obtain pure A.

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