Question

Calculate the expected freezing point of a solution of 0.579 g pelargonic acid in 4.225 g of lauric acid, if the melting point of pure lauric acid was measured to be 43.20*C. (MM pelargonic acid = 158.2 g/mol; MM laruic acid = 200.32 g/mol). Kf for lauric acid is 3.90*C/Mc; unit answers are in *C

I have tried solving for Mc, which = 11.076923 mole/kg (Mc)
and tried getting just moles for lauric acid by multiplying 11.07692 mol/kg x 1kg/1000g x 4.225g = 0.0468 moles

I am unsure where to go from here. Please help.

Answer 1

I think you're on the wrong track.
Kf has the units of *C/molal

pel = pelargonic acid
lau = lauric acid

mols pel = grams/molar mass = ?
m = molality pel = mols/kg lau = ?
dT = Kf*m. You know Kf and m, solve for dT. Subtract dT from 43.2 to arrive at the new freezing point.