Question

Menthol is a crystalline substance with a peppermint taste and odor. When 0.960 g of menthol is dissolved in 25.0 g of cyclohexane, the freezing point of the solution is lowered by 5.12 °C. The freezing point and Kf constant for cyclohexane can be found here. Calculate the molar mass of menthol.

Answer 1

SInce image not uploaded regarding kf value ,I take web value of 20

We have formula dT = i x Kf x m

since compound is organic i =1 , dT = Freezing point change = 6.59-0.1 = 6.49 C

Now 6.49 = 1 x 20 x m

m = molality = 0.3245 = moles of solute / SOlvent mass in Kg

0.3245 = moles of solute / 0.9882

moles of solute = 0.320671 = Mass of solute /Molar mass of solute

0.320671 = 28.7 /Molar mass

Molar mass = 89.5 g/mol

Now we were given percentage of mass , we divide each by respective atomic masses

C = 38.7/12 = 3.225 , H = 9.74/1 = 9.74 , O= 51.55/16 = 3.222

divide eah by smallest number

C/O = 3.225/3.222 = 1 , H/O = 9.74/3.222 = 3 , O/O = 1

now emperical formula is CH3O , its formula mass is 12 + 3 + 16 = 31

now let N be number of repeating units

Molar mass = emperical mass x N

89.5 = 31 x N

N = 3 ( rounded value should be taken)

Hence moleclar formula is   C3H9O3