Crystallization Study Questions
1) What is the purpose of a crystallization?
2) You want to purify 10 g of benzoic acid that has been contaminated with 0.2 g of salicylic acid. Solubilities in water of the two compounds are given in the following table
3) You want to purify 10 g of benzoic acid that has been contaminated with 10 g of salicylic acid. (Consider the table given in question 2).
4) A compound A has the following solubility properties:
You have a sample of 0.1 g of compound A which is contaminated with a compound B.
(If you include multiple crystallizations in your answer, use just enough hot solvent in each step to completely dissolve compound A.)
5) Listed below are solubility vs temperature data for an organic compound in water.
6) You have 2 g of benzhydrol and have been advised to recrystallize it from hexanes. How much hexanes will you use to recrystallize this product?
7) Suppose you are recrystallizing a compound and boil the solution for so long that a substantial amount of the liquid evaporates. What is likely to happen to some of the solute? What should you do if this occurs?
8) Suppose you have prepared a compound which is reported in the literature to have a pale blue color. When dissolving the substance in hot solvent prior to recrystallization, the resulting solution is blue. Should you use decolorizing charcoal before allowing the hot solution to cool? Explain your answer.
9) Each of the following compounds, A-D, is equally soluble in the three solvents listed. In each case, which solvent would you choose? Explain.
10) Suggest possible crystallization solvents for the following compounds.
11) A student was recrystallizing a compound. As the hot solution cooled to room temperature, no crystals appeared. The flask was then placed in an ice-water bath. Suddenly a large amount of solid material appeared in the flask. The student isolated a good yield of product, however, the product was contaminated with impurities. Explain.
12) A student used benzene to recrystallize a compound. As the hot solution cooled to room temperature, very few crystals appeared. The flask was then placed in an ice-water bath. Suddenly a large amount of solid material appeared in the flask. Then, the student filtered the solid with vacuum, but only a few crystals remained on the filter paper. Explain these results.