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Solubility problem.

Discussion in 'DAT Discussions' started by JohnDoeDDS, Jul 19, 2006.

  1. JohnDoeDDS

    JohnDoeDDS Senior Member
    7+ Year Member

    Dec 25, 2004
    Likes Received:
    Ok guys can someone please explain this. For some reason I am totally confused by the Kaplan explanation. thanks.

    The solubility of Fe(OH)3 in an aqueous solution was determined to be
    4.5 * 10^-10 mol/l. What is the value of the Ksp for Fe(OH)3.

    Could youu please explain this in detail so I understand how to solve these types of problems? I remember I used to get these right in gen chem but for some reason am really confused by this now.

    Also my math skills without a calc are horrible so if some could also explain how to solve this by working the scientific notation I would really appreciate it. Thanks guys!
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  3. dat_student

    dat_student Junior Member
    5+ Year Member

    Apr 4, 2005
    Likes Received:
    Fe(OH)3 --> Fe3+ + 3 OH-
    ................. S.........3S

    Ksp = [Fe][OH-]^3

    Ksp = S * (3S)^3

    Ksp = 27 S^4 = 27 (4.5 * 10^-10)^4 = 4.5^4 * 27 * 10^-40 = 1.1 * 10^-36

    check my math
  4. 714guy

    714guy counting down till 5/2012
    7+ Year Member

    Feb 26, 2004
    Likes Received:
    Dental Student
    first setup the balanced equation

    Fe(OH)3 --> Fe(3+) + 3 OH(-)

    4.5 * 10^-10 M of Fe(OH)3 disassociates into 4.5 * 10^-10 M of Fe and 3*(4.5 * 10^-10 M) of OH

    Ksp = [Fe][OH]^3

    now plug and jug . . .

    For the OH value you have to multiply it by 3 b/c of stoichi number, for every Fe there are 3 OHs

    Ksp = [4.5 * 10^-10 M][3*4.5 * 10^-10 M]^3

    = [4.5 * 10^-10 M][1.35 X10^-9]^3
    * I got the 3* junk by just 4.5 X 3 = 13.5 then you move the decimal foward one so you reduce the exponent by 1, so from -10 to -9

    = [4.5 *10-10][1.3s^3x10^-27]
    * for this I just used the xponent rules and 9*3, turns out to be a very small number. then multiplying the two will give you (4.5 * 1.35^3 ) X 10^-37.

    from that you should be able to figure out the answer from the choices.
  5. jeffrey

    jeffrey Junior Member

    Nov 20, 2005
    Likes Received:

    Dude, no offense, but perhaps you could be a little bit more proactive and go to your undergraduate library (or one nearby if you are no longer a student, or order books from Amazon etc.) and look through a General College Chemistry text book when ever you encounter difficulties solving gen chem questions, or questions in orgo, math, bio etc., etc., etc. There are literally hundreds of text books out there that will help much more than Kaplan or another student on this website would be able to.

    Good luck

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