Submitted by Cameron Gren / University of North Alabama on Thu, 07/11/2013 - 13:32
We do a freezing point depression experiment (naphthalene in cyclohexane from Beran), but it just doesn't work! We have cu down on the amount of solute, but the students just cannot obtain consistent data! I'm about ready to scrap it, but I would really like them to do SOMETHING with colligative properties in lab. Any suggestions would be most welcome!
Sabrina Sobel / Hofstra University


We have 2 labs in our in-house published Gen Chem lab book. One is freezing point depression of t-butanol by addition of organic acids, that works well, and the second is a survey of colligative properties: Tyndall effect, Sterno, ammonium chloride smoke, and emulsions. I can scan the pages for you and email them for you to review.

Thu, 07/11/2013 - 14:30 Permalink
Cameron Gren / University of North Alabama

Thanks, that would be great! We actually do a lab on colloids right after the fp depression experiment that we affectionately call "the slime lab." They make a PVA emulsion then mix it with a borax solution. The borax cross-links the PVA chains and makes a nice gooey slime. We have them calculate the viscocity by timing a falling lead sphere through the slime. Then we let them take it home if they want to! It's always a crowd pleaser.

Thu, 07/11/2013 - 17:03 Permalink
Dave Frohnapfel / Slippery Rock University of PA

We perform the cyclohexane FP depression experiment each Spring term to good results using either naphthalene or biphenyl as a solute.  We use 10 mL of cyclohexane and about 0.200 g solute using nested test tubes and a coil of copper wire around the precision thermometer for stirring.  We have found that the key for success is clean equipment.  Prior to the week of the experiment - I have a TA clean all tubes and wire coils with brush/acetone.  The largest source of contaminant is the wire coil which is difficult to rinse well.  Students have to be reminded to thoroughly rinse the equipment after use. Naphthalene is a better solute since the odor will let you know if you are using a dirty setup.  Students using dirty equipment will see a lowering of the FP of the "pure" solvent as well as a steady decrease while freezing since then are really testing a solution at that point.  If the contamination isn't too bad, they can often achieve acceptable results since the change in FP from weak solution to stronger solution is still proportional to the concentration.  

We run about 9 sections of 24 students split into groups of 3 and (provided the students work with care), most of them produce resuls within 10 g/mol of the chosen solute.  

Fri, 07/12/2013 - 08:21 Permalink