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Today:. Your experiences with recrystallization? Mixed-solvent recrystallization MiniQuiz. Table for Part A Solubility of Pure Benzoic Acid Which solvent(s) is/are suitable for recrystallization?. Rt. H 2 O. Hot. Cooled. Rt. 95% EtOH. Hot. Cooled. Rt. Warm. Pet Ether. Cooled. Rt.

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Today:

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  1. Today: • Your experiences with recrystallization? • Mixed-solvent recrystallization MiniQuiz

  2. Table for Part A Solubility of Pure Benzoic AcidWhich solvent(s) is/are suitable for recrystallization? Rt H2O Hot Cooled Rt 95% EtOH Hot Cooled Rt Warm Pet Ether Cooled Rt Acetone Warm Cooled

  3. Part B Recrystallization of Impure Benzoic Acid Your sample contained benzoic acid (70%), oxalic acid (30 %) and a trace of a dye. Role of “active charcoal”? Today: Obtain mp. of purified benzoic acid (range!) and of your original impure benzoic acid.

  4. Questions: • What do you expect the purities of Crop 1 and Crop 2 to be? Same? Different? What melting point ranges do you expect for Crop 1, Crop 2, original mixture? Explain. • What is the “mother liquor”? • What compound(s) do you expect to be in the mother liquor after filtering off your first crop of benzoic acid crystals? • Is it possible to retrieve 100% of solid sample in a recrystallization? (Assume that you are a very experienced chemist.)

  5. Part C Recrystallization of Naphthalene from a solvent pair melting point? • Often it is impossible to find one good solvent for recrystallization. Then a solvent pair has to be used: • Solvent (#1) should dissolve compound at RT. • Solvent (#2) should NOT dissolve compound at RT. • Both solvents must be miscible!

  6. Methanol-Water Ethanol-Water Acetic acid-Water Acetone-Water Ether-Methanol Ether-Acetone Ether-Petroleum ether Toluene-Ligroin Methylene chloride-Methanol Common Solvent Pairs for Crystallization

  7. Do solubility tests for Naphthalene: • H2O, 95% EtOH, Pet Ether, and Acetone • Goal is to find a good “solvent pair”: Solvents #1 and #2 Procedure: Part C:

  8. 2) Once you have chosen the solvent pair, then use ~ 0.5 g of Naphthalene in 50-mL E-flask. (Record the exact weight to three decimal places 0.000 g) 3) Dissolve compound in minimum amount of warm solvent #1(“good solvent”), start by using 10 mL of solvent #1. Record additional mL of solvent #1 used. Use 20 drops per 1 mL, when using a disposable glass pipette and a small rubber bulb. Crystals must be just dissolved; do not use a great excess! 4) Solvent #2 is added dropwise to the warm mixture, until mixture just becomes cloudy. Cloudiness indicates precipitation. At this time, add solvent #1, just until the cloudiness disappears. The solution is now saturated. Cool solution to obtain crystals.

  9. Reading Solubility Data Example: Solubility of Benzoic Acid Solubility in water(g/l) at 0o: 1.7 at 95o: 68.0

  10. Next time:Molecular Modeling(no Notebook Preparation required, but do preview/prestudy Experiment)

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