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October 30, 2014

Homework Help: ORG CHEM LAB

Posted by Anonymous on Monday, February 26, 2007 at 10:25pm.

Im doing a conclusion on a lab of acetaminophen. We were to take a melting point of the crude smale of acetaminophen and melting point of the crystallized acetaminophen.

The mp for crude sample: 169.2 degree C
The mp for crystallized sample: 172. degree C

Why the change in mp's? Is it because the second mp was less pure?

There was also a difference in the % yields. The crystallized sample % yield was less than the crude sample, why?

IF ANYONE could help me answer this, I would greatly appreciate it!

Think this thing through!!
Remember from freshman chemistry about depression of melting point. To refresh your memory, perhaps you will recall that when a non-volatile solute (such as an impurity) was added to a non-volatile solvent (such as PURE acetominophen), the melting point of the pure sample was lowered. Isb't that what your data showed?

Wouldn't you EXPECT the yield to be lower for the pure sample as opposed to the impure sample. That's what recrystallization is all about. You want to place the crude sample in a solvent, somtimes at a slightly elevated temperature, let the solution cool, and let the PURE crystals recrystallize, leaving the IMPURITIES behind in the solvent. You get less PURE material because the impurities have been left behind.

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