Question

Consider that you prepared a solution by mixing 0.14 g organic solute with 7.81 g of cyclohexane. If you measured that the freezing point of the solution was 4.86 oC, what is the molar mass of the organic solute? Cyclohexane has a freezing point of 6.55oC and Kf= -20.2 °C/m.

Express your answer in units of g/mol to to three significant figures.

Answer 1

a)           Find the molality of the solution :

∆ Tf = Kf m

There fore molality (m) =∆ Tf /Kf

∆ Tf= 6.55 deg- 4.86 deg= 1.69 deg

Kf= -20.2 °C/m.

molality = 1.69/20.2 = 0.08366 mol

Number of moles of solute= molality x kgs of solvent

Weight of solvent cyclohexane = 7.81 g =0.00781kg

=0.08366 mole solute x 0.00781kg = 0.000653 mol

Molar mass of solute = weight/ no of moles

Here weight of the organic solute =0.14 g

There fore molar mass =

0.14 g /0.000653 =214.4g/mole