1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Chemistry, colligative properties-freezing point

  1. Dec 13, 2008 #1
    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


    Answer is -7.18 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. jcsd
  3. Dec 13, 2008 #2


    User Avatar

    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)?
  4. Dec 13, 2008 #3
    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?
  5. Dec 13, 2008 #4


    User Avatar

    Staff: Mentor

    No. Write equation of dissociation reaction and use the definition you have already posted.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Chemistry, colligative properties-freezing point
  1. Colligative Properties (Replies: 1)