Question

Menthol is a crystalline substance with a peppermint taste and odor. When 0.533 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 2.84 ∘C. Look up the freezing point and f constant for cyclohexane in the Colligative Constants table. Calculate the molar mass of menthol.

Answer 1

Answer: The molar mass of menthol is 156.15 g/mol

Explanation:

Depression in freezing point is given by:

$\Delta T_f=K_f\times m$

$\Delta T_f=T_f^0-T_f=2.84^0C$ = Depression in freezing point

$K_f$ = freezing point constant = $20.8^0C/m$

m= molality

$\Delta T_f=K_f\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

Weight of solvent (cyclohexane)= 25.0 g = 0.025 kg  

Molar mass of solute (menthol) = ?

Mass of solute (menthol) = 0.533 g

$2.84^0C=20.8\times \frac{0.533}{xg/mol\times 0.025}$

$x=156.15g/mol$

The molar mass of menthol is 156.15 g/mol