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Homework Help: Another solubility problem

  1. Oct 10, 2007 #1
    I don't know why i am getting this wrong

    Question :
    Rank the following five salts in order of decreasing solubility, in terms of mass per unit volume.
    (The most soluble gets rank 1, the least soluble gets rank 5.)

    PbCl2 (ksp=1.60E-5)
    Ca5(PO4)3OH (ksp=6.80E-37)
    Ag3PO4 (ksp=1.80E-18)
    Sr3(PO4)2 (ksp=9.00E-15)
    Hg2CO3 (ksp=1.00E-31)

    My answer

    for each of the salt above, i found their molar masses
    PbCl2 (278.106 g/mol)
    Ca5(PO4)3OH (502.307 g/mol)
    Ag3PO4 (418.574 g/mol)
    Sr3(PO4)2 452.80 g/mol)
    Hg2CO3 (461.188g/mol)

    also for each of the salt i found the concentration of it in mol/L using each of the give Ksp.
    for example, Ksp of PbCl2 = [PB2+][Cl-] = (x)(2x)^2 -----> find x and that will be the [PbCl2]

    After that i would use that concentration in mol/L and multiplied it by the molar mass of PbCl2 to get the solubility in grams/ L

    for each the salts, i did the same method as above... then i ranked them from highest solubility in g/L to lowest solubility........

    my answer was

    1 PbCl2
    2 Ca5(PO4)3OH
    3 AgPO4
    4 Hg2CO3
    5 Sr3(PO4)2

    I am still not getting the correct answer.. what am i doing wrong ? did i mis-understood the question and ranked the salts in the opposite order or ?
  2. jcsd
  3. Oct 11, 2007 #2


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    I see a problem here (bolded)... Look again at the definition of Ksp.
  4. Oct 11, 2007 #3
    Ksp of PbCl2 = [PB2+][Cl-]^2 = (x)(2x)^2 , where x is the concentration of Pb2+, also the concentration of PbCl2 and 2x is the concentration of Cl-

    is that correct?
  5. Oct 11, 2007 #4


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    It looks right to me. Sorry, I didn't see what you had defined 'x' as in your previous post. If your math was accurate, it should give you the correct answer.
  6. Oct 11, 2007 #5
    for one of the salts (in the same question) that i had to rank is Hg2CO3
    I don't know if my equation was correct since i still couldn't get rank the salts correctly

    so for Hg2CO3 salt...i wasn't sure which equation is the right one

    Hg2CO3(s) <-----> 2Hg+ + CO3^2- (molar ratio would be 2:1)
    Hg2CO3(s) <-----> Hg + HgO + CO2 (if i use this equation, what is my Ksp product going to be? ksp=[Hg][HgO][CO2]? or is it just ksp=[Hg2+][CO2-]? molar ratio 1:1??

    i have no idea how to write the Ksp equation for Hg2CO3. Maybe this is the problem why I can't rank the salts correctly
  7. Oct 12, 2007 #6


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    What order did you rank them and what was the answer?
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