Question

Menthol is a crystalline substance with a peppermint taste and odor. When 1.36 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 7.27 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of menthol.

Kf= 20.8 C/m

Freezing point = 6.59 C

Answer 1

Answer – Given, mass of methanol = 1.36 g , mass of cyclohexane = 25.0 g

Freezing point solution is lowered = 7.27 °C , Kf= 20.8 C/m ,

freezing point = 6.59^oC ,

we know the ∆Tf = 7.27 °C

We know the formula for the freezing point depression

∆Tf = Kf*m

So, m = ∆Tf / -Kf

          = 7.27 ^oC / 20.8 ^oC.m^-1

          = 0.349 m

We know the molality = moles of solute / kg of solvent

So, moles of solute = molality * kg of solvent

                                  = 0.349 m * 0.025 kg

                                   = 0.00874 moles

So, molar mass of methanol = 1.36 g / 0.00874 moles

                                             = 155.6 g/mol