Question

Benzene has a normal freezing point of 5.50°C and a molal freezing point depression constant of 5.12°C/m. A solution prepared by dissolving 0.335g of compound X in 5.00g of benzene has a freezing point of 1.34°C. Assuming that compound X is a nonelectrolyte, what is the molar mass of compound X?

Answer 1

delta Tf = 5.50 oC - 1.34 oC

= 4.16 oC

we have below equation to be used:

delta Tf = Kf*mb

4.16 = 5.12 *mb

mb= 0.8125 molal

mass of solvent = 5.00 g

= 5*10^-3 kg [using conversion 1 Kg = 1000 g]

we have below equation to be used:

number of mol,

n = Molality * mass of solvent in Kg

= (0.8125 mol/Kg)*(0.005 Kg)

= 4.063*10^-3 mol

mass of solute = 0.335 g

we have below equation to be used:

number of mol = mass / molar mass

4.063*10^-3 mol = (0.335 g)/molar mass

molar mass = 82.5 g/mol

Answer: 82.5 g/mol