Recrystallization Handout

Recrystallization Handout - Experiment 2: Recrystalliza4on...

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Unformatted text preview: Experiment 2: Recrystalliza4on Calibra4on Curve Ques4on Using the calibra4on curve shown below, the correct mel4ng point for a sample that has an observed mel4ng point of 100 C is A.85 C B.92 C C.100 C D.108 C E.120 C Calibration Curve 130 120 110 100 90 80 70 60 50 40 40 50 60 70 80 90 100 110 120 Observed Melting Point Literature Melting Point Mel4ng Point Ques4on You have a mixture composed of 90% benzoic acid and 10% naphthalene. Benzoic acid has a melting point of 122.4 oC, while naphthalene has a melting point of 80.2 oC. You can say with certainty that the melting point of the mixture will be: A. B. C. D. E. greater than any of the two melting points. less than the melting point of naphthalene less than the melting point of benzoic acid halfway between the two melting points cannot be determined without measuring Mel4ng Point Composi4on Diagram Recrystalliza4on Recrystalliza4on: a method of _______ of a solid compound in which the solid (solute) __________________, then forms crystals again as ______________ How does this process purify the solid? A 7-Step Program 1. Choose the appropriate solvent or solvent pair. 2. Dissolve the solute. 3. Decolorize the solu4on. 4. Filter any suspended solids. 5. Crystallize the solute. 6. Collect and wash crystals. 7. Dry crystals. Finding a Perfect Match:The Ideal Solvent Remember that recrystalliza4on is separa4on of a solid mixture! Leave impuri4es behind when dissolving (___________)! OR Leave impuri4es behind when crystallizing (______________)! Solvent should dissolve solute when _____ but not when _________. Solvent should not ______ with solute. Solvent should be nonflammable, nontoxic, inexpensive, and reasonably vola4le (evaporates!). When One Solvent Isn't Good Enough: Solvent Pairs A ______________________ can be used when one solvent is not suitable. Solvent pairs must be ___________ (very soluble in each other). Usually, solvent A dissolves solute very well. Solvent B does not dissolve solute well. (Think about polarity.) Pairs are usually found by experiment (we've already found a pair for your experiment!) Recrystalliza4on Ques4on Ader the crystals are collected by filtra4on ader a recrystalliza4on, the crystals are washed with cold solvent because: A. washing with hot solvent would redissolve the solid, giving a lower yield B. the cold solvent removes any material that is not part of the crystal lafce C. you want the crystals to be dry, so you wash it D. the washing cleans the filter funnel for the next person E. a and b ...
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