I'm a little bit confused here. I assume you are adding glycerin to water in order to lower the freezing point? Tf is supposed to be the change in temperature of the freezing point? If this is the case then your Tf should be -3.8, (freezing point changed from 0 for pure water to -3.8 for the solution).
Once you have the correct Tf you can calculate your molality using Km, which is just a constant for different solutions that you can look up online. It looks like you've already done this given that you have a molality, but if you used 13 for Tf I'm thinking yours may be incorrect.
This looks like it was from a chem lab, I hope you recorded the how much water that you used? In order to solve from here you really need to give us this information. You would want to use your molality that you calculated in order to calculate number of mols of glycerin. Do this by the equation molality = mols of glycerin / (Kg of water, or whatever solvent you actually used).
You should now have mols of glycerin. Calculate molar mass by just doing 2.84gm / number of mols.
Calculate % error by looking up the actual molar mass of glycerin and doing ( ( |Actual Molar Mass| - |Experimental Molar Mass| ) / Actual Molar Mass ) * 100
This post was edited by RzChaos on May 5 2013 06:16am