Question

a 0.497g sample of an unknown compound lowers the freezing point of 50.0g of cyclohexane by 0.12^OC. Calculate the molecular weight of the compound.

Answer 1

We know that ΔT_f = K_f x m
Where

ΔT _f = depression in freezing point

        = freezing point of pure solvent – freezing point of solution

        = 0.12^oC

K _f = depression in freezing constant of cyclohexane = 20.2 ^oC/m

m = molality of the solution

    = ( mass / Molar mass ) / weight of the solvent in Kg

   = (0.497 g / M ) / 0.050 kg                      50.0 g = 50.0/1000 kg = 0.05 kg

   = 9.94 / M

Plug the values we get

0.12 = 20.2 x(9.94 / M )

M = 1673.2 g/mol

Therefore the molar mass of the unknown compound is 1673.2 g/mol