I worked this problem, I would like to know if I did it correctly, I think I missed something, I'm not sure.....



When a 20.0 gram sample of an unknown compound is dissolved in 500 grams of benzene, the freezing pointof the resulting solution was observed to be 3.77 degrees C. If the freezing point of pure benzene is 5.48 degrees C and the Kf fro benzene is 5.12 degrees C/m, calculate the molar mass of the unknown compound.

ÄTf = KfM
Kf for benzene = 5.12 degrees C/m
the normal freezing point = 5.480C
ÄT = (5.48-3.77) = 1.71

1.71 degrees = (5.12 degrees C/m)(20.0g/MW)(5kg of solvent)
MW = 20.48g/mol

Is this correct?

delta T is correct at 1.71 C.

Then delta T = Kf*m and
m = molality = 1.71/5.12 = 0.334 mols/kg
mols = g/MW
0.334 = 20/MW/0.5 kg or
0.334 = (20/MW)*(1/0.5kg)
MW = ??
I have something like 120 or so. Check my thinking. Check my arithmetic.

ok got it!!!!

Thanks for clearing it up for me!

To determine if your calculation is correct, let's go through the steps:

1. Calculate the change in freezing point (ΔTf):
ΔTf = Normal freezing point - Observed freezing point
ΔTf = 5.48°C - 3.77°C
ΔTf = 1.71°C

2. Calculate the molality (m):
molality (m) = ΔTf / Kf
m = 1.71°C / 5.12°C/m
m ≈ 0.3343 mol/kg

3. Calculate the moles of solute (n):
moles (n) = m * kg of solvent
n = 0.3343 mol/kg * 0.500 kg
n ≈ 0.1671 mol

4. Calculate the molar mass of the unknown compound (MW):
MW = mass of the compound / moles of the compound
MW = 20.0 g / 0.1671 mol
MW ≈ 119.7 g/mol

Based on the calculations, it seems that you made a slight mistake. The correct molar mass of the unknown compound is approximately 119.7 g/mol, not 20.48 g/mol as you calculated.