# Homework Help: Chemistry, colligative properties-freezing point

1. Dec 13, 2008

### Flip

The problem statement, all variables and given/known data

A solution is made by dissolving 250.0 g of solid potassium chromate in 1.00 kg of water. What will be the freezing point of the new solution?

molal freezing point-depression constant of water = 1.86 degrees Celsius/molality
molality = mol/kg

Relevant equations

[Delta]T(freezing point)=(Van't Hoff Factor)(molal concentration of solute particles)(molal freezing point-depression constant)

Van't Hoff Factor = (moles of particles in solution/moles of solute dissolved)

The attempt at a solution

(250 g K2CrO4)(1 mol K2CrO4/194.188 K2CrO4) = 1.29 mol of K2CrO4
(1.29 mol K2CrO4/1.00 kg H2O) = 1.29 mol/kg
Change in freezing point = (1.29 molality)(1.86 degrees Celsius/molality)
Change in freezing point = 0 degrees celsius - 2.40 degrees celsius
Change in freezing point = -2.40 degrees celsius

Since ionic compounds rarely dissociate completely the Van't Hoff Factor has to be used. Except I don't know how to use it...

Last edited: Dec 13, 2008
2. Dec 13, 2008

### Staff: Mentor

This is false statement. Simple ionic salts almost always dissociate 100%.

What is van't Hoff factor for this salt (assuming 100% dissociation)?

3. Dec 13, 2008

### Flip

The Van't Hoff Factor isn't given, which is why I thought that I had to find it myself. But if it does dissociate completely then doesn't that mean that the Van't Hoff Factor would just be 1?

4. Dec 13, 2008

### Staff: Mentor

No. Write equation of dissociation reaction and use the definition you have already posted.