[genghiscohen]

Hello Chemical Forums,

I had a lab that started out with a 30mL solution of unknown composition. The goal was to find out which two compounds were in the mixture (out of a given list of seven) and their ratio by doing a fractional distillation and then GC.

In the distillation I found that the first compound had a BP of 56ºC (corresponds to acetone on the list, next closest in BP is about 8ºC above) and the second compound had a BP of 80ºC (corresponds to cyclohexane on the list, next closest is +/-11ºC). At this point I had three fractions—A, B, and C—and I headed over to the GC machine.

I then shot two µL of fraction A by itself. Then 1µL of A +1µL of acetone. Then 2µL of B. Then 2µL of C. Then 1µL of C + 1µL of cyclohexane.

Here are the chromatograms (this links to a photobucket account where the jpgs can be viewed):

http://s1153.photobucket.com/albums/p516/notescopier/

So I think the first problem is that my A compound has less acetone than it does the second compound. To me the "A + acetone" verifies the identity of the compound as acetone, revealing a problem in that the acetone peak is the smaller of the two peaks in area and height.

Problem two is most likely the bigger issue. The chromatogram for C shows what I would expect, that the majority of the sample is composed of the "0.75" compound that was presumed to be cyclohexane. But then the chromatogram for "C + cyclohexane" shows that the peak in the mid-sixties grows significantly in relation and not the peak around 0.75.

Any advice on what I've done wrong here? During the distillation my second leveling occurred at 80ºC, right where our MSDS says cyclohexane begins to boil. No other compound on the list came anywhere near close to this boiling point.

Any help or advice is greatly appreciated. I'll be returning often, but thanks to everyone in advance

[genghiscohen]

To provide a little follow up (the moderator can close this topic after today if they'd like), I went to one the TA's office hours today and we reached the conclusion that fraction C was shot with acetone by mistake.

The end.