1. Why is it necessary to allow enough time for your samples to dry? 2.What is t

Question

1. Why is it necessary to allow enough time for your samples to dry?

2.What is the optimal amount of solvent to add during a recrystallization? Why?

3. How does rate of crystal growth effect the purity of a recrystallized compound?

4. After recrystallizing an impure sample of benzoic acid, a student scraped it off the filter paper and took a melting point. Most of the compound melted between 121 and 122oC. A small amount remained un-melted even at temperatures above 200oC. Explain this behavior.

Explanation / Answer

1. Enough time must be given to the sample for drying so as to get dried sample with no solvent molecules so as to obtain good saturated solutions using the minimum amount of the same or another solvent for recrystallization.

2. In order to maximize your purity, you’d like to use enough solvent to dissolve the crystals and keep the impurities in solution even after cooling. You’d like to minimize the amount of solvent used so that as little as possible sample remains in solution after cooling. The best way to accomplish both goals is to use enough solvent so that it can dissolve everything while hot, but to use no more than the minimum required so as to maximize yield after cooling. The minimum required is when your solution is just barely at the saturation point while boiling hot. Use just enough solvent so that the material is just barely soluble, or is just a little cloudy to show that it’s just barely saturated.

3. Recrystallization is a method of purifying a solid. The crystallization process itself helps in the purification because as the crystals form, they select the correct molecules, which fit into the crystal lattice and ignore the wrong molecules. This is of course not a perfect process, but it does increase the purity of the final product.

Rate of crystal growth affects the purity of a recrystallized sample. It should be slow because slow crystal growth means each of the atoms/molecules can enter the selected site with ease. Only atoms/molecules that fit the size and shape will attach.

4. This happens because the obtained recrystallized sample which was melted was not 100% pure and contained some inpurities, which might have been introduced by scraping it of from the filter paper.. impurities increase the melting point of a pure sample.

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