Calculate the freezing point in C of a solution

Question

Calculate the freezing point in oC of a solution that contains 23.2 grams of Cr(NO3)3 dissolved in 72.0 grams of pure H2O. The freezing point and molal freezing point constant, Kf, of pure H2O are 0.00 oC and 1.86 oC/m , respectively.

Answer 1

First we have to find the molaltiy of the solution.

23.2g *(mol/238g)  .0975 moles Cr(NO3)3

Next we need to find the number of moles of solute particles after the Cr(NO3)3 dissolves.

.0975 moles Cr(NO3)3 *(4 solute particles/molecule) = .390moles of solute particles

Next I calculated the molality of solute particles.

m= (moles of solute)/(kg of solvent)

m= .390moles/.072kg = 5.42m

The freezing point depression has a magnitude proportional to the molal concentration of solute particles.

freezing point of solvent - freezing point of solution = Kf *(m)

0C - freezing point of solution = (1.86C/m) *(5.42m)

freezing point of solution = -10.1C

As a rule the freezing point of a solution is always lower than that of the solvent.