Question

1. What is the purpose of crystallization in an organic chemistry procedure?

2. Using the solubility data you found for benzoic acid, calculate the volume of water required to dissolve 1.0 g of benzoic acid at room temperature. Calculate the volume of boiling water needed to dissolve 1.0 g of benzoic acid.

3. Explain why a Büchner or Hirsh funnel is used to isolate the final crystallized product instead of stem funnel.

4. Explain when a mixture of solvents would be used to carry out a recrystallization instead of a single solvent.

5. Assume that you are given 1 g of an unknown compound, which could be acetanilide or phenacetin. The solubility of the two organic compounds, acetanilide and phenacetin are given below. You used to enough water to recrystallize phenacetin but later on you found out that the unknown given to you was acetanilide. Comment on whether or not this would have affected your percent recovery of the unknown after recrystallization and if so, how.

Substance	Formula Wt.	m.p. oC	Solubility, cold water	Solubility,boiling water
Acetanilide	135.2	114	0.54 g/100 mL	5.0 g/100 mL
Phenacetin	179.2	135	0.076 g/100 mL	1.22 g/100 mL

6. Give three criteria for a good recrystallization solvent.

7. You want to purify 10 grams of benzoic acid that has been contaminated with 0.2 g of salicylic acid. You have found the following data by looking at the Merck Index:

Compound	Solubility in water at 20 oC (g/10 mL)	Solubility at 100 oC (g/10 mL)
Benzoic Acid	0.029	0.680
Salicylic Acid	0.22	6.67

a) What volume of Boiling water is needed to dissolve the 10 g of benzoic acid? (show calculations)

b) How much benzoic acid will crystallize after cooling to 20o C (show calculations)

c) Will any salicylic acid crystals form? Why or why not? (show calculations/ reason for answer)

d) What is the maximum amount of benzoic acid that could be recovered in the first crop of this recrystallization? (Show calculations)

e) Will the benzoic acid be pure? Why or why not?

Answer 1

Answer 1

Crystallization is a method of purification of organic compounds. It is capable of removing both soluble as well as insoluble impurities.

In this method, firstly crude product is dissolved in minimum amount of suitable solvent. The solution is filtered to remove insoluble impurities.

Then, the filtered solution is warmed and then filtered through Buchner funnel. This allows fast filtration and then the filtrate is allowed to evaporate on its own. The pure compound will crystallize leaving the soluble impurities in the solvent.