[ElementGirl12]

Hi All,

My labs in O-chem have not been turning out correctly.  We've had 3 labs so far and the percent error have been really high.  The first lab was Recrystallization, where we started with a mass of product A, mixed it with other products, then we were supposed to separate A, recrystallize it and make it pure again.  At the end, my lab group was supposed to end with product A and it should have the same mass or very close to it. But the mass was way less, and we also took the melting point to show how pure our product was at the end.  It was extremely higher than what product A should be.  It wasn't even close to the other products we mixed with product A.  Our professor let us attempt it again. This time, product A was a higher mass, with the same extremely high B.P.  Our other labs were extraction(separatory funnel) then recrystallizing the product we want, and synthesis of Aspirin. Each time we're not getting a pure sample at the end. It's frustrating b/c our grade is based on our final numbers.  We're not rushing, we went extremely slow in our lab re-do to make sure we followed the directions to a T. I don't know what the problem could be.

The labs aren't faulted, b/c I overheard the teacher telling another group good job, and they were getting an A. So obviously, theirs turned out right.  Does anyone know what we could try differently with these kind of labs to make us get the same product at the end? Any help would be appreciated! Our next lab is extracting caffeine from No Doz, so its going to be same thing...trying to get a pure sample of caffeine.

[Arkcon]

Its hard to tell from your posting what might be wrong, so we can't really help.  All I can say is be sure that you've followed the method detailed in the lab practical properly.  Be sure your partners are helpful, and not cutting corners, and not bullying you to do the same.

Maybe next time, write out each step, and confirm for us if its really what you did, or if you think you've missed something.

[Borek]

another group good job, and they were getting an A

Why don't you discuss the lab with them? Go over your procedure and minute details of what you did.

[orgopete]

I can sympathize with you about correlating results with the grade. When I taught, I wanted to disconnect chemistry from 'cooking'. Rather than letting students expect what their result should be, I tried to use different unknowns and to try to get student to draw conclusions from their results. That isn't the case here.

Obviously I cannot know what may have been done in this instance, but we can reason that if you recovered all of the material, the mp would be depressed while the weight would be the highest. If you succeeded in removing all of the impurities and in the process of dissolving them also dissolved some of your product, the weight would be less, but the purity and mp would be higher. Based upon my experience, I'd say you applied too much heat in taking the mp. The amount of heat required to melt a very small sample is less than the amount of heat for the thermometer. I'm presuming you had the actual sample and thus the observed mp could not be elevated, so the difference had to come from the mp determination. If too much heat were applied, you may see the sample melt, you realize "Oh, oh, it's melting", turn off the heat and record the temperature the thermometer reaches, it would be too high. What might have been done is that now the temperature of the apparatus is in the mp range, repeat the mp (or use the apparatus just after someone else and before it has time to cool).