Posted by SuSu on Tuesday, February 2, 2010 at 12:52am.
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, Tuesday, February 2, 2010 at 1:02am
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, Tuesday, February 2, 2010 at 1:06am
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, Tuesday, February 2, 2010 at 1:20am
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.
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