Explain how to properly perform a recrystallization.
Added by Jenny C.
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The solvent should dissolve the compound when hot but not when cold. It should also not dissolve impurities at any temperature or dissolve them very well at all temperatures. Show more…
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1. What is the purpose of recrystallization? 2. How are insoluble impurities removed during a recrystallization? 3. How are soluble impurities removed during a recrystallization? 4. What properties are desirable in a recrystallization solvent? 5. Give two reasons why vacuum filtration is sometimes preferable to ordinary gravity filtration. 6. During a crystallization procedure, the mass of the crude sample was 56.4 g and the mass of purified crystals was 45.3 g. Calculate the yield or percent recovery of the procedure.
Adi S.
7. PURIFICATION OF AN UNKNOWN Recall the seven-step recrystallization procedure: 1. Choose the solvent. 2. Dissolve the solute. 3. Decolorize the solution (if necessary). 4. Filter suspended solids (if necessary). 5. Recrystallize the solute. 6. Collect and wash the crystals. 7. Dry the product. You will purify an unknown provided by your instructor, 2.0 g if working on a macroscale and 100 mg if working on a microscale. Conduct tests for solubility and the ability to recrystallize in several organic solvents, solvent pairs, and water. Conserve your unknown by using very small quantities for the solubility tests. If only a drop or two of solvent is used, heating the test tube on a steam bath or a sand bath can evaporate the solvent, and the residue can be used for another test. If decolorization is necessary, dilute the solution before filtration. It is very difficult to filter a hot, saturated solution from decolorizing carbon without recrystallization occurring in the filtration apparatus. Evaporate the decolorized solution to the point of saturation and proceed with the recrystallization. Submit as much pure product as possible with evidence of its purity (i.e., the melting point). From the posted list, identify the unknown. If an authentic sample is available, your identification can be verified by a mixed melting point determination (see Chapter 3). Cleaning Up. Place decolorizing charcoal, if used, and filter paper in the nonhazardous solid waste container. Put organic solvents in the organic solvents waste container and flush aqueous solutions down the drain. RECRYSTALLIZATION PROBLEMS AND THEIR SOLUTIONS
Madhur L.
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