Question

Menthol is a crystalline substance with a peppermint taste and odor. When 1.48 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 7.90 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of menthol.

Answer 1

Answer:

Molar mass of menthol is 155.86 g/mol

Explanation:

Let's apply the colligative property of freezing point depression:

ΔT = Kf . m . i

As menthol is an organic compound we assume the i (Van't Hoff factor as non electrolytic, so i = 1)

m = mol /kg (molality)

Kf = Cryoscopic constant, in this case, for cyclohexane

ΔT = T° freezing of pure solvent - T° freezing solution

7.90°C = 20.8°C / m . m . 1

7.90°C / 20.8 m/°C = m

0.380 m = molality

0.380 moles are the moles of solute (menthol) dissolved in 1kg of solvent, but our mass of solvent is 25 g.

Let's convert the g of solvent to kg to work with molality

25 g = 0.025 kg

molal . solvent mass = moles of solute

0.380 mol /kg . 0.025kg = 9.49×10⁻³ moles

Now, we have the moles and the mass of solute. Let's determine the molar mass of menthol (grams / moles)

1.48 g / 9.49×10⁻³ m = 155.86 g/m