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The Perils of Ksp

  1. Mar 28, 2014 #1

    Qube

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    Gold Member

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

    http://i.minus.com/jfSl42bdxVH40.png [Broken]


    2. Relevant equations

    pHO = -log[HO^-]

    3. The attempt at a solution

    1) Sulfide ion is a strong base so sulfide ion does not exist in water. Sulfide ion reacts with water upon contact to an extent of 100%. The reaction of sulfide ion with water forms hydroxide ion and hydrogen sulfide ion. So that teacher's work above is already wrong without any need for calculation as the teacher purports that there is a measurable quantity of sulfide ion in the system at equilibrium. [S^2-] should be ~0.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Mar 29, 2014 #2

    Borek

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    Staff: Mentor

    Sure it does, especially at high pH. With pKa2=12 (more or less) at pH 12 [HS-]=[S2-]. Not that it matters much here, but if you want to to stick to the facts - do it correctly.

    Yes, exact calculations should take sulfide hydrolysis into account. It also should take into account AgOH precipitation, plus formation of several complexes (like Ag(OH)2-) and so on. You won't be able to solve this problem by hand. Or, rather, it is doable, but it will take you several days.

    My bet is that the problem at hand was selected just to show general method of finding molar solubility from the Ksp, and hydrolysis was ignored to keep it simple.

    Feel free to try to solve the problem taking hydrolysis into account. Be ready for a frustrating evening.

    It is already calculated as 10-17. That's ~0.
     
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