Practice question

Question

An electrolyte solution was prepared by dissolving 40.0 g of KCl (molar mass 74.55g) in 2.000 kg of water. According to this data, the freezing point of this solution is:

Answer 1

The formula required to answer this problem is ΔT = iK_fm where i is the van't Hoff factor, K_f is the freezing point depression constant of water and m is the molality of the solution.

Since one molecule of KCl will dissolve into one potassium ion and one chloride ion, the van't Hoff factor is 2.

K_f for water is 1.86^oC/m

The molality of the solution is found by dividing the number of moles of KCl by the kg of solution.

m = (40.0g/74.55g*mol^-1) / 2.000 kg = 0.268 m

So... plugging into the original formula

ΔT = iK_fm = (2) * (1.86^oC/m) * 0.2680 m = 0.997 ^oC

This is NOT the new freezing point since ΔT was calculated. This is merely how much the freezing point of water will be depressed. Since the freezing point of water is 0^oC then the new freezing point is 0 - 0.997 which is -0.997 ^oC