Menthol is a crystalline substance with a peppermint taste and odor. When 0.606

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.

(°C/m)

Normal freezing

point (°C)

(°C/m)

Normal boiling

point (°C)

Answer:_____g/mol

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

Explanation / Answer

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 T = Kf x molality

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

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

M = (Kf x w ) / (T 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

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