Posted by SuSu on .
Does anyone know how to do this problem, i can't seem to solve it.
If 39.25 g of a substance is dissolved in 155.5 g of benzene solvent, the freezing point of the solution is 0.08 oC. Calculate the apparent molar mass (g/mol) of substance.

Chemistry 
DrBob222,
delta T = Kf*molality.
molality = moles solute/kg solvent.
moles = g/molar mass.
Post your work if you gst stuck.
Use equation 1 to solve for molality.
Use equation 2 to solve for moles solute.
Use equation 3 to solve for molar mass. 
Chemistry 
SuSu,
so this is what i did,
0.08C = (5.065Ckg/mol)(x/0.1555) which is 0.002456071.
i then took 39.25g/0.002456071 which is 15980 g/mol.
This seems to big for molar mass of a substance 
Chemistry 
DrBob222,
Mistake 1. delta T is not 0.08. The problem states that the freezing point of benzene is 0.08 C. You must have or must look up the normal freezing point of benzene and subtract 0.08 to arrive at delta T.
#2. I found 5.12 for the Kf for benzene and the units are oC/m. I don't understand the units you have but the number you have used is not that much different than what I used..
I ran through the problem VERY roughly and obtained something in the neighborhood of 240 for the molar mass.