Calculate the freezing point of a solution containing 0.5 g KCl (Molar mass = 74.5 g/mol) dissolved in 100 g water, assuming KCl to be 92% ionized. Kf of water = 1.86 K kg / mol.
The percentage of dissociation is 92%. Therefore the degree of dissociation α is 0.92. The total number of moles after dissociation is 1+ α = 1.92 = i. The term i is also the ratio of calculated molar mass to the theoretical molar mass. We know the calculated molar mass, the theoretical molar mass is calculated as:
i = 
1.92 = 
Theoretical molar mass = 38.802
Using this and the given data, the depression in the freezing point can be calculated from the formula
ΔTf = 1000 x Kf x 
x W1
= 1000 x 1.86 x 
x 100
= 0.23K.
The freezing point of the solution is calculated as
ΔTf = Initial freezing point – the freezing point after addition of solute
0.23 = 0 – the freezing point after addition of solute
The freezing point of the solution after the addition of KCl = -0.23°C.
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