In: Chemistry
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 – 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 = 6.59oC ,
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