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Homework Help: Chemistry- Selective Precipitation

  1. Oct 20, 2007 #1
    1. The problem statement, all variables and given/known data

    My molarity is wrong which keeps me from ding the rest of this problem...any help would be greatly appreciated.

    A researcher has a 450.0 mL solution that is 0.421 M in Hg2(NO3)2 and 0.0191 M in Pb(NO3)2. She plans to add solid potassium iodide to separate the mixture by selective precipitation.

    Ksp PbI2 = 1.4e-8
    Ksp Hg2I2 = 1.2e-28

    What is the maximum concentration of iodide ions that may be in solution to achieve the best separation possible?

    M [I-]= ?

    How many grams of potassium iodide should she add to achieve this?

    g of KI

    What will the percentage of mercury(I) ions remaining in solution be?

    % Hg2+2

    2. Relevant equations
    ksp= ...

    3. The attempt at a solution
    hg2i2 = hg2 + 2i
    1.2e-28 = [.421]^2
    = 1.68e^-14, which is wrong and I have no idea what I am doing wrong
  2. jcsd
  3. Oct 20, 2007 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Ksp=??? Can you write down the expression for a general compound AmBn?

    This calculation is not correct. First write down the correct expression for Ksp, then plug in numbers.
  4. Oct 21, 2007 #3
    thanks, but I already figured that out
    I failed to realize it said MAXIMUM
    so would be ksp for pbi
    = 8.6e-2
    but now I am having problems with b and c
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