Question

QUESTION: How do you determine the molar mass of the unknown, nonelectrolyte compound for each trial?


This is for a lab. My data is:

freezing point of solvent (water): -0.58
freezing point of unknown: -0.93
Mass of solvent (water): 19 ml
Mass of unknown: 1.045g

I know the formula to use is Tf=Kfm

So plugging that in, I got:

0.34=1.86*m
m=0.183
0.183=moles of solute/.019 kg
moles of solute = 3.477e-3
molar mass = 1.045g/3.477e-3
molar mass = 300.55

But I don't think this fits any of the possible compounds. Those being: Urea (60 g/mol), 2-deoxy-D-ribose (134 g/mol), D-ribose (150 g/mol), glucose (180 g/mol), sucrose (342 g/mol), alanine (89 g/mol), glycine 75 g/mol), or phenylalanine (165 g/mol).

I guess it could be sucrose? I don't think the data is bad, because the graph did the supercooling thing, and I did two trials of freezing the water. So the data should be right.

Can you see anything that I might have done wrong in my calculations?

Answer 1

First, the formula to use is delta T = Kf*m and delta T is 0.35 (not 0.34) Follow that through and you get about 292 for the experimental molar mass. None of the answers match. I think the freezing point of pure water at -0.58 C sounds fishy (it should be zero) BUI thermometers CAN be off by that much, I guess. After all, it's the DIFFEREMCE that counts and that difference is 0.35. I don't see anything wrong with your calculations; it has to be the data. One thing to remember is that this method of determining the molar mass is not and EXACT procedure. It can be off by 50% or so depending upon a lot of things. For non-volatile compounds,however, it is better than that but not error free. I would try decreasing/increasing those those molar mass of the compounds listed by +/- 10% and see if that gets close. For example 10% of 342 for sucrose is 342-34 = about308. So I might go with sucrose, also, especially when we consider that the 1.86 is an average of all compounds + you know there is the possibility of experimental error.