The following questions relate to crystallization techniques. a) Which of the fo
ID: 499963 • Letter: T
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The following questions relate to crystallization techniques. a) Which of the following would be the best single solvent for the crystallization of an impure sample? b) Given the solubility information above, which combination of solvents should be used for a mixed solvent crystallization (assume all of these solvents are miscible). Briefly describe this technique. c) Rather than attempting filtration to remove the liquid from a collection of crystals, a student decides to let the solvent evaporate off. Is this a problem? Explain. d) A student attempts to crystallize a sample from methanol. All of the sample dissolves in hot solution, but no crystals (or any precipitate) were obtained after cooling. What could the student try to get crystals? e) A student weighs the product obtained from crystallization of an impure sample using methanol/water as the crystallization solvent. This student finds that the crystals weigh more than the impure sample (> 100% recovery). Assuming no errors were made in weighing the samples, what is the most probable source of error?Explanation / Answer
(a). B would be a good solvent for recrystallization
(b) A+ B would be a good solvent mixture for recrystallization.
Procedur : Add the hot solvent B dropwise to the crude so that all the compound dissolves completely (do not add excess) then cool the solution to room temperature. At this point add solvent A dropwise till the solution becomes cloudy. Then again warm the solution to make it transperent. Allow to cool slowly to room temperature . when it comes to room temperature keep it in ice bath to complete the recrytallization process.
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(c) This would be wrong. As if the student removes the solvent , then all the impurities which are disoolved in solvent will precipitate together with the pure compound.
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(d) The student has used too much sample. concentrate the solution by evaporating some of the solvent (not all) and then try to cool it slowly.
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(e) Crystals are not dry.
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