1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Density and freezing point

  1. Feb 3, 2007 #1
    Assume that the density of water is 1.00 g/ml . Calculate the quantity of KBr that would need to be added to lower the freezing point of 535 ml of water by 1.40 degree celsius:

    What I'm trying to do is the following:

    Step 1:

    Tf = Kf . m

    I took Tf = 1.4
    Kf = 1.86
    and found the molality as 0.75

    Step 2:

    then I used the molality equation which is

    molality = moles solute/kg solvent

    molality = 0.75
    kg solvent = 0.535 kg
    and got the no. of moles as 119 g/mol

    Step 3

    no. of moles = mass /molar mass and
    got 47.6 g for mass

    but the answer is still wrong ... I don't know where I'm making mistake. It's an online assignment so I don't know what the answer should be. But please guys help me and check if I'm doing everything right.
  2. jcsd
  3. Feb 3, 2007 #2
    what is the vanthoff factor for KBr????
  4. Feb 3, 2007 #3
    oh so you mean to say that I do have to consider vanthoff factor for KBr too in this problem?
    I must have forgotten about that?
  5. Feb 3, 2007 #4
    vanthoff factor for KBr is 2 since we get two ions from one mole of KBr.
    Is that right?
  6. Feb 3, 2007 #5
    Hi, I'm doing the caculations again, see if they are right now,
    I'm getting molality as 0.376
    no. of moles =0.20116
    molar mass =119 g/mol
    mass = 23.9 g
  7. Feb 3, 2007 #6
    hey, thanks for reminding me about the i value .... I tried it in the assignment questions and i got the right answer.
    Thanks a lot for the help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook