Question

Menthol is a crystalline substance with a peppermint taste and odor. When 0.606 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 3.23 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of menthol.

Solvent	Formula	Kf value* (°C/m)	Normal freezing point (°C)	Kb value (°C/m)	Normal boiling point (°C)
water	H2O	1.86	0.00	0.512	100.00
benzene	C6H6	5.12	5.49	2.53	80.1
cyclohexane	C6H12	20.8	6.59	2.92	80.7
ethanol	C2H6O	1.99	–117.3	1.22	78.4
carbon tetrachloride	CCl4	29.8	–22.9	5.03	76.8
camphor	C10H16O	37.8	176

Answer:_____g/mol

Answer 1

Given that mass of menthol w = 0.606 g

molar mass of menthol M = ?

mass of solvent ( cyclohexane ) W = 25 g = 0.025 kg

Kf of cyclohexane = 20.8 ^oC/m

depression in freezing point dT = 3.23 ^oC

We know that

   depression in freezing point ΔT = Kf x molality

molality = (w/M) x (1/W in kg)

Then, depression in freezing point ΔT =   Kf x { (w/M) x (1/W) }

M = (Kf x w ) / (ΔT x W)

=  (20.8 ^oC/m x  0.606 g) / (3.23 ^oC x  0.025 kg)

= 156

M = 156

Therefore,

molar mass of menthol = 156