Chemistry 234
Organic Chemistry Laboratory

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Experiment 2: Recrystallization (10 points)


You will be learning several techniques for purification of organic compounds. One very effective way to purify solids is through recrystallization. In this experiment, you will be given an impure sample of benzoic acid that you will purify by recrystallization. You will also determine the melting point of the recrystallized benzoic acid to assess the purity.

Impure sample of benzoic acid.
Benzoic acid after recrystallization.



The procedure for recrystallization is found in your text, page 96. You will make the following changes to the procedure:

1. Scale up the procedure so that it accommodates 2 grams of benzoic acid.

2. Weigh the sample of impure benzoic acid before you begin recrystallization.

3. Skip the "Decoloration" section.

Heat the sample with enough water to dissolve all soluble material.

Add more water if needed.

Filter the hot solution to remove insoluble material

Boil off some of the water.

Use a boiling stick when you boil off the solvent.

Be sure to use the GLASS powder funnel for your hot filtration rather than the plastic funnel.

Crystals form as solution is cooled.

Cool to complete crystallization.

Filter, wash, and dry the product.

Note: You will need a total of 40-50 mL of water for this experiment. If you add too much water you can boil some off after filtration.

Report Requirements

Your lab report should include the following:

1 . References to procedure with any changes noted

2. Weight of recrystallized benzoic acid

3. Percent recovery of benzoic acid

4. Melting point of recrystallized benzoic acid and literature melting point

5. A description of the purified benzoic acid

6. Interpretation of data and short conclusion (discuss purity, recovery, and overall effectiveness of recrystallization)

7. Answer these questions in your report:

a. Why did you do a hot gravity filtration?

b. Why did you do a cold filtration?

c. The solubility of benzoic acid in water at 0 C° is 0.020 g/100 mL of water. If you used 3.0 g of impure benzoic acid and 100 mL of water, what is the maximum yield of benzoic acid that could be recovered?

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