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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.
     
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