Calculate the freezing point of a solution made by dissolving 3.50 g of potassium chloride ( = 74.55 g/mol) in 100.0 g of water. Assume ideal behavior for the solution; K_f = 1.86°C/m.

Question

Quizplus · Accepted Answer

First, we need to calculate the molality of the solution:
$$
molality = \frac{mol~of~solute}{kg~of~solvent} = \frac{3.50~g~KCl}{0.100~kg~H_2O}\cdot \frac{1~mol~KCl}{74.55~g~KCl} = 0.469~mol/kg
$$
Next, we can use the freezing point depression equation:
$$
\Delta T_f = K_f\cdot molality
$$
$$
\Delta T_f = 1.86°C/m\cdot 0.469~mol/kg = 0.87434°C
$$
The freezing point of pure water is 0°C, so the freezing point of the solution is:
$$
freezing~point = 0°C - \Delta T_f = 0°C - 0.87434°C = -0.87434°C \approx -0.9°C
$$
Therefore, the best choice is (A).

Calculate the Freezing Point of a Solution Made by Dissolving