Homework Help: Chemistry

Posted by Paul on Thursday, April 2, 2009 at 4:29pm.

Aspirin can be prepared by the reaction of salicylic acid with an excess of acetic anhydride, using phosphoric acid as a catalyst. Once the aspirin is formed, it is precipitated from the reaction mixture by adding water and filtering. The solid aspirin obtained in this way is contaminated with acetic acid. To purify it, it will be recrystallized from an ethanol-water mixture. Recrystallization is a commonly-used method for the purification of solids. In this case it is done by dissolving the impure aspirin in a hot ethanol-water mixture; purified aspirin will recrystallize upon cooling the solution.

If the recrystallization step was skipped, would this result in an increase or decrease of the apparent percent yield or have no effect?

I feel that it would result in an increase in the apparent percent yield because the actual yield would be larger than what it would have been had the recrystallization been carried out.

Is this reasoning correct?

Also, from the above description of the experiment, what does "The solid aspirin obtained in this way is contaminated with acetic acid" mean in simple terms?

Chemistry - DrBob222, Thursday, April 2, 2009 at 5:03pm
I feel that it would result in an increase in the apparent percent yield because the actual yield would be larger than what it would have been had the recrystallization been carried out.

I think your reasoning is ok and the answer is ok. If you have used all of the salicylic acid, then the only thing left is acetic acid and product. Technically, one might be able to air/heat dry the material to drive off the acetic acid and the yield would be better because of the increased amount of the product. You ALWAYS lose some product in a crystallization. What would bother me more, but the problem doesn't address it, is that some of the salicylic acid may not have reacted so the product could be contaminated with some of the starting material, also, which would increase the apparent yield, too. From the statement in the problem, the contamination with salicylic acid has not been addressed; it assumes, I suppose that all of the S.A. will be used and the only contaminant is the excess of acetic acid used in the course of the experiment.

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