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Homework Help: Solubility with Common Ion

  1. Mar 31, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the solubility of Mg(OH)2 in a solution buffered at pH 8.80.

    2. Relevant equations

    3. The attempt at a solution
    The answer is .45 M or 26 g/L but I can't get that for some reason.

    I can find the concentration of hydroxide [OH-] = 6.3 x 10-6
    Mg(OH)2 -> Mg+2 + 2OH-
    ................0+x,....6.3x10-6 + 2x

    1.8x10-11 = x(6.3x10-6 + 2x)^2

    x = 1.65 x 10-4 M (neglecting the 6.3 x10-6)
    I've also tried solving the cubic with similar results.
    Does anyone where I went wrong?
  2. jcsd
  3. Apr 1, 2007 #2


    User Avatar

    Staff: Mentor

    Solution is buffered. [OH-] is not 6.3x10-6 + 2x but JUST 6.3x10-6.
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