2) Pretend that you are in the lab, and the instructor has given your group a vi

Question

2) Pretend that you are in the lab, and the instructor has given your group a vial containing crystals of benzoic acid mixed with crystals of sodium chloride. The task is to isolate pure crystals of benzoic acid. One team member suggests using tweezers and a microscope, another member says that she can use ethyl acetate to separate the benzoic acid from the salt, and a third team member says that water could be used instead of ethyl acetate or tweezers.

2a) Using only ethyl acetate as a solvent, describe a laboratory procedure step-by-step to obtain pure benzoic acid from the mixture. (Remember: the final product must be a solid! FYI: ethyl acetate is the solvent used in non-acetone nail polish remover; it’s relatively non-toxic and evaporates quickly.)

2b) Using only water as a solvent, explain step-by-step how to obtain pure solid benzoic acid from the original mixture of NaCl and benzoic acid.

2c) Which method (tweezers, ethyl acetate, or water) is fastest overall? Why?

Explanation / Answer

2 a)

1.Obtain a sample of an unknown mixture. Record the number of the vial.

2. Weigh the unknown mixture in the vial on an analytical balance. Transfer the contents of your vial to a 100 mL beaker. Weigh the empty vial.

3. Add 20 mL ethyl acetate to the beaker. Stir to dissolve the unknown mixture. Use a glass funnel to transfer the solution of unknown in ethyl acetate to a 125 mL separatory funnel on a ring. Rinse the beaker with two 10 mL portions of ethyl acetate, and add it to the separatory funnel.

4. Add 10 mL aqueous 1.5 M NaOH and shake the separatory funnel. Withdraw the aqueous layer. Repeat with a second 10 mL portion of NaOH.

5. Add 10 mL distilled water to the organic layer in the separatory funnel. Shake the separatory funnel well. Withdraw the aqueous layer. Aqueous layers from steps 4 and 5 may be combined in a beaker labeled “Aqueous Layer.” Save this for later extraction of the acidic component.

6. a. Dry the organic layer in the separatory funnel by shaking with 15 mL saturated NaCl solution. b. Drain the lower aqueous layer and discard.

7. a. Drain the organic layer from the separatory funnel into a 125 mL Erlenmeyer flask. b. Add anhydrous Na2SO4 drying agent until no more clumping is observed. Set the flask aside for several minutes with occasional swirling. c. Filter this through a fluted filter paper into a tared (pre-weighed) 100 mL round bottom flask. Rinse the Erlenmeyer flask with 3-5 mL of pure ethyl acetate and pour this rinse onto the same filter.

8. Remove the solvent using the RotoVap.

9. Weigh the round bottom flask containing the solid neutral component.

10. Determine the melting point of the compound.

Aqueous Layer

1. Add concentrated HCl dropwise to the beaker labeled “Aqueous Layer” until the solution is acidic to litmus. Then add a few drops of acid in excess. Check for complete precipitation.

2. Add 30 mL ethyl acetate and extract.

3. Drain the aqueous layer into a waste beaker.

4. a. Dry the organic layer in the separatory funnel by shaking with 15 mL saturated NaCl solution. b. Drain the lower aqueous layer and discard.

5. a. Drain the organic layer from the separatory funnel into a 125 mL Erlenmeyer flask. b. Add anhydrous Na2SO4 drying agent until no more clumping is observed. Set the flask aside for several minutes and swirl as before. c. Filter this through a fluted filter paper into a tared (pre-weighed) 100 mL round bottom flask. Remember to rinse the Erlenmeyer flask as in step 7c above.

6. Remove the solvent using the RotoVap.

b)

1. Transfer the sample of benzoic acid and sodium chloride to one of the 250 ml beakers.

2. Add 75 ml of water.

3. Stir the mixture to dissolve the salt.

4. Prepare an ice water bath in the 1-liter beaker. Place the 250 ml beaker in the ice water bath, but without allowing it to tip over or take in water from the ice bath. Continue to stir the mixture.

5. Place a piece of filter paper in the funnel, and dampen it slightly so it adheres to the funnel. Place the empty 250 ml beaker beneath it, and pour the mixture through the filter paper in the funnel. The benzoic acid, which did not dissolve, will remain in the filter paper, while the sodium chloride solution will pass through.

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