Technique, Recrystallization of Solids : 9. A compound you have prepared is repo

Question

Technique, Recrystallization of Solids

: 9. A compound you have prepared is reported in the literature to have a pale yellow color. When the substance is dissolved in hot solvent to purify it by crystallization, the resulting solution is yellow. Should you use decolorizing charcoal before allowing the hot solution to cool? Explain your answer.

10. An organic chemistry student dissolved 0.30 g of a crude product in 10.5mL (the minimum amount required) of ethanol at 25oC. He cooled the solution in an ice-water bath for 15 minutes and obtained beautiful crystals. He filtered the crystals on a Buchner funnel and rinsed them with about 2.0mL of ice-cold ethanol. After drying, the weight of the crystals was found to be 0.015 g. Why was the recovery so low?

Explanation / Answer

9)

Decolorizing charcoal is used toremove unwanted colored impurities from a solution, and does notdiscriminate against any particular colored compounds present insolution.

So long as the compound you wish to purify exerts acertain pigment (in this case yellow), it would be wise not to usedecolorizing charcoal because that would in effect remove a goodchunk of your desired compound, leaving you with a lower percentyield.

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