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

compound solubility at 20° C (g/10 mL) solubility at 100°C (g/10 mL)
benzoic acid 0.029 0.680
salicylic acid
 0.22 6.67
  1. What volume of boiling water is needed to dissolve the 10 g of benzoic acid?
  2. How much benzoic acid will crystallize after cooling to 20°C?
  3. Will any salicylic acid crystals also form?
  4. What is the maximum amount of benzoic acid that can be recovered in the first crop of this recrystallization?
  5. Will the benzoic acid be pure?

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).

  1. What volume of boiling water is needed to dissolve the 10 g of benzoic acid?
  2. How much benzoic acid will crystallize after cooling to 20°C?
  3. What amount of salicylic acid crystals will also form?
  4. Will the benzoic acid be pure?

4) A compound A has the following solubility properties:

solubility at 25°C
solubility at 100°C
0.01 g/mL
0.1 g/mL

    You have a sample of 0.1 g of compound A which is contaminated with a compound B.

  1. If compound B is completely insoluble in water and 2 mg of compound B is present, how could you purify Compound A?
  2. If compound B has the same solubility behavior as A and 2 mg of this compound is present, how could you purify compound A? Would one recrystallization produce absolutely pure A?
  3. Assume compound B has the same solubility behavior as A and 25 mg of this compound is present. Would one crystallization produce absolutely pure A? How many crystallizations would be needed to produce pure A? How much A would have been recovered when the crystallizations have been completed?

    (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.

temp.
sol. in 10 mL water
0
0.15 g
20
0.30 g
40
0.65 g
60
1.10 g
80
1.70 g
  1. Using the data in the above table, graph the solubility of the compound vs temperature. Connect the data points with a smooth curve.
  2. Suppose 0.1 g of this compound is mixed with 1.0 mL of water and heated to 80° C. Would all of the compound dissolve? Explain.
  3. The solution prepared in (b) is cooled. At what temperature will crystals of the compound appear?
  4. Suppose the cooling described in (c) were continued to 0° C. How many grams of A would come out of solution?

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.

  1. Compound A: benzene, acetone, or chloroform
  2. Compound B: carbon tetrachloride, methylene chloride, ethyl acetate
  3. Compound C: methanol, ethanol, or water
  4. Compound D: ethanol, acetone, or diethyl ether

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.

Answers to these questions.