Question

A 0.991g sample of an unknown was dissolved in 10.8 g cyclohexane. If the freezing point depression of the solution was 4.17°C, find the molar mass of the unknown. See Table 1 (in your lab book) for freezing point information concerning cyclohexane.

Answer 1

Mass of sample = 0.991 g

Mass of cyclohexane = 10.8 g   ( cyclohexane is solvent )

Delta Tf = 4.17 deg C

We use

Delta Tf = m x kf

Here kf is the freezing point constant of solvent.

Kf for cyclohexane = 20 deg C/ m

m is molality of the solute .

Delta Tf is the freezing point depression.

m = Delta Tf / kf = 4.17 C / 20 C per m

= 0.2085

Now molality = number of moles / mass of solvent in kg.

Mass of solvent in kg = 10.8 g x 1 kg / 1000 g = 0.0108 kg

0.2085 m = Number of moles / 0.0108 kg

Number of moles of solute = 0.002252 mol

Now number of moles = mass in g / molar mass

Molar mass = mass in g / number of moles

= 0.991 g / 0.002252 mol

= 440.1 g/mol

So the molar mass of given compound would be = 440.1 g /mol