Question

A solution is prepared by dissolving 0.24 g of a solute in 12.7 g of cyclohexane.

What is the freezing point change, rT_f?

Round off answer to to one place after the decimal: __ . __ °C

(MM, molar mass of solute = 128.19 g/mol)

Answer 1

Solution :-

mass of solute = 0.24 g

mass of cyclohexane = 12.7 g * 1 kg / 1000 g = 0.0127 kg

Freezing point change = ?

formula to calculate the change in freezing point is as follows

Delta Tf = Kf*m

where delta Tf = change in freezing point

Kf = molal freezing point constant

m= molality

lets calculae moles of the solute

moles = mass / molar mass

            =0.24 g /128.19 g per mol

           = 0.0018722 mol

now lets calculate the molality

molality= moles of solute / kg solvent

             = 0.0018722 mol / 0.0127 kg

             = 0.147417 m

Now lets calculate the change in the freezing point

Kf of cyclohexane = 20.0 degree C/m

Delta Tf= Kf*m

             = 20.0 C per m * 0.147417 m

            = 2.9 C

Therefore the change in the freezing point = 2.9 ^oC