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PH of molecular salt

  1. Feb 2, 2014 #1

    Qube

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    Gold Member

    1. The problem statement, all variables and given/known data

    Find the pH of 0.20 M Iron (III) sulfate.

    2. Relevant equations

    The metal ion becomes hydrated (water is a ligand) and forms iron hexahydrate.

    The sulfate ion is a weak base.

    3. The attempt at a solution

    Now, in considering the pH of this solution, should I consider the below reaction even though it's small extent? This reaction consumes the acid, but the reaction extent is small!

    [itex]Fe(OH_{2})_{6}^{3+} + SO_{4}^{2-} \leftrightharpoons HOSO_{2}^{-} + Fe(OH_{2})_{5}(OH)^{2+}[/itex]

    Or should I ignore that reaction and only consider the below reaction?

    [itex]Fe(OH_{2})_{6}^{3+} + H_{2}O \leftrightharpoons H_{3}O^{+} + Fe(OH_{2})_{5}(OH)^{2+}[/itex]
     
  2. jcsd
  3. Feb 3, 2014 #2

    Borek

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    Staff: Mentor

    Question is slightly ambiguous.

    When you dissolve Fe3+ some of it will precipitate as Fe(OH)3, substantially lowering solution pH (check Ksp). Question doesn't say whether 0.20 M was the initial concentration of the sulfate, or is the final concentration after the precipitation. Not that the difference will be large.
     
  4. Feb 3, 2014 #3

    Qube

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    Gold Member

    Good idea. We haven't covered solubility products yet; that's the next chapter. I'll just ignore precipitation but that is a valid concern. The 0.2M is referring to initial concentration of sulfate.

    However, is the first, small extent reaction worth considering?
     
  5. Feb 3, 2014 #4

    Borek

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    Staff: Mentor

    I would ignore sulfate and concentrate on the Fe3+ behavior, as it is much stronger acid than the sulfate anion is a base.

    Unfortunately, this is kind of a problem that is hard to answer without guessing what the teacher had on mind [PLAIN]http://www.bpp.com.pl/IMG/grumpy_borek.png. [Broken]
     
    Last edited by a moderator: May 6, 2017
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