Question

Menthol is a crystalline substance with a peppermint taste and odor. When 1.22 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 6.52 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of menthol.

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

        = 6.52 ^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

   = (1.22 g / M ) / (25.0x10^-3 Kg)

Plug the values in ΔT _f = K_f x m       we get

                          6.52 = 20.2 x (1.22 g / M ) / (25.0x10^-3 Kg)

                               M = 151.2 g/mol

Therefore the molarmass of menthol is 151.2 g/mol