Question

A solution containing 0.050g of an unknown nonelectrolyte in 2.50g of cyclohexane was found to freeze at 5.1 degrees C. What is the molar mass of the unknown substance?

Please show the work, thanks.

Answer 1

Given freezing point = 5.1 C

mass of unknown non electrolyte = 0.050 g

but actual freezing point of cyclohexane is 6.59 C

depression in freezing point dF = 6.59 - 5.1 = 1.49 C

Kf = 20.8

depression in freezing point = Kf x m

1.49 = 20.8 x m

m = 0.0716

we know that

molality m = moles of solute x 1000 / mass of solvent (g)

0.0716 = moles of unknown substance x 1000 / ( 2.5 g)

moles of unknown susbstance = 1.79 x 10-4

molar mass = mass / moles

molar mass of unknown substance = 0.050 / 1.79 x 10-4

molar mass of unknown susbtance = 279.33 g/mol