Molar mass of an unknown substance question?

Question

A 0.2436-g sample of an unknown substance was dissolved in 20.0 mL of cyclohexane. The density of cyclohexane is 0.779g/mL. The freezing-point depression was 2.5ºC. Calculate the molar mass of the unknown substance. (Kf cyclohexane = 20.0ºC/m)

Answer 1

Mass cyclohexane = 20.0 mL x 0.779 g/mL = 15.58 g=>0.01558 Kg

delta T = Kf x m

2.5 = 20.0 °C/m x m

m = molality solution = 0.125

0.125 = moles solute / mass solution

0.125 = moles solute / 0.01558 Kg

moles solute = 0.00195

molar mass = 0.2436 g / 0.00195 mol =125.1 g/mol