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Homework Help: Ksp - Prelab help

  1. Aug 2, 2006 #1
    Hi, I'm trying to do my prelab and I have no idea where to start... if someone can point me to the right direction, I will absolutely be grateful... :redface:

    Given the following equilibrium:

    PbI2 <----> Pb + 2I

    You have a solution of the following:

    4.77 ml of .00274M Lead Nitrate solution
    4.35 ml of .00215M Iodide solution
    5.00 ml of water

    You measured the solution and get the following:
    %T blank = 100.3 %
    %T sample = 42.9 %
    Absorbance of a 1.000 mM Iodide soln = .775

    Calculate the following:
    micromoles of Pb(II) originally put in solution
    micromoles of I- originally put in solution
    mM of I- at equilibrium
    micromoles of I- in solution at equilibrium
    micromoles of I- precipitated
    micromoles of Pb(II) precipitated
    micromoles of Pb(II) in solution at equilibrium
    mM of Pb(II) in soluion at equilibrium

    The only ones I solved that were correct were the following:
    total volume = 14.12 ml
    absorbance of sample = .3688

    Hope someone can help me... :cry:
  2. jcsd
  3. Aug 2, 2006 #2
    first: your reaction equation is not completely correct. Dissolving [tex] PbI_{2}[/tex]:

    [tex] PbI_{2} \longrightarrow Pb^{2+} + 2 I^{-}[/tex]

    for the original micromols [tex]Pb^{2+}[/tex] you have to use the formula [tex] c = \frac {n} {V}[/tex] and you use the given: 4.77 ml of .00274M Lead Nitrate solution

    The same counts for micromols [tex]I^{-}[/tex]

    The third one you need to use the law of Lambert-Beer: [tex]A =\epsilon\cdot c\cdot l[/tex]. From the given : Absorbance of a 1.000 mM Iodide soln = .775, you can calculate [tex]\epsilon[/tex] And then with this found [tex]\epsilon[/tex] you can calculate [tex] [I^{-}]_{eq}[/tex]. Only you need to know l: the length of the cuvet where the light goes through. Since it's a prelab, you'll probably have this parameter.

    Then you have everything to go on calculating [tex]K_{sp}[/tex]
    Last edited: Aug 2, 2006
  4. Aug 2, 2006 #3

    Thank you sooo much! :rofl:

    Okay, I've solved everything correctly except for Ksp :surprised

    I should use Ksp = [Pb]^2 right?

    But which concentrations should I use? The ones at equilibrium? Do I need to do "ICE"? I'm not familiar with it, but I searched and they mentioned "ICE", not sure if it's applicable here though...

    Thanks again!
    Last edited: Aug 2, 2006
  5. Aug 2, 2006 #4

    yes, but watch your notations !

    [tex] K_{sp} = [Pb^{2+}(aq)][I^{-}(aq)]^{2}[/tex]

    you already know the equilibrium concentrations and you need them here. As for your question about ICE: you already had to use it to get to the equilibriumconcentrations.
  6. Aug 2, 2006 #5

    Thank you, thank you, thank you~~sdekivit!! I finally got it :rolleyes:

    Also yes, I should learn how to use the notations correctly... I'll read up on how to use the notations here on the forum :smile:
    Thank you again!!!!
  7. Apr 16, 2007 #6

    Hi, i couldn't help but wonder how you calculated the equilibrium concentration of I- from the molar extinction coefficient, E?? I have a very similar problem to Rachelle's post...
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