Question

A student performs this experiment but does not have the flask boiling when dissolving the original sample. All other steps are performed correctly (using the minimum amount of solvent to dissolve as much of the sample as possible, etc.). What will be the effect on this student’s eventual percent recovery after recrystallization as a result of this mistake? Explain in one sentence.

A student performs this experiment but forgets to rinse the product after vacuum filtration. Which type(s) of impurity would likely be present the following week? Choose among: “soluble”, “insoluble”, “both”, or “neither”, and explain in one sentence.

Answer 1

(a) As in experiment student does not boiled the flask during dissolution that means the solution is in saturated or unsaturated state. For maximum recovery after recrystallization we need a solution to be supersaturated. When we boil a mixture the solubility increase and at cooling it again the solution become supersaturated. this supersaturated solution can recover 100 percent of solute after recrystallization.

The student's mistake will lead to decrease in percent recovery after recrystallization because it need to add more solute at ice bath temperature to induce crystallization.

(b) The washing of crystals after vacuum filtration is performed to avoid soluble impurities . Insoluble impurities are already washed off after dissolution step by sieve filtration before recrystallization step. Washing after crystallization step is a neccessory step to avoid the soluble impurities that would set on the crystals when the solvent evaporates.