Question

Suppose that a student calculated Kf of biphenyl to be 81.2C kg/mol. calculate the mass of an unknown if a solution containing 1.64 g of unknown in 18.22 g of biphenyl froze at 39.9 C

Answer 1

Read your question and rephrase it to take care of the errors. You surely don't mean calculate mass. The mass is given. And you don't give the normal freezing point so we don't know delta T. And that 81.2 kg/mol? Kf is usually given in C/m.

Answer 2

Its asking for the molar mass of the unknown, sorry.

Answer 3

I don;t know what 81.2 kg/mol means for a Kf. I don't know the normal freezing point of biphenyl.

Answer 4

Here is how you do it if you find that information.
delta T = (39.9-normal f.p.) = Kf*m
Substitute and solve for m = molality

m = mols solute/kg solvent
Substitute and solve for mols solute.

mols solute = grams solute/molar mass solute. You know mols and grams, solve for molar mass.

But you need the normal freezing point and a viable Kf.

Answer 5

183.696 grams