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Homework Help: Selective Precipitation Chemistry Problem

  1. Oct 20, 2013 #1
    1. The problem statement, all variables and given/known data
    A solution of [itex]AgNO_{3}[/itex] is added to a solution containing 0.100 M [itex]Cl^{-}[/itex] and 0.100 [itex]CrO_{4}^{2-}[/itex].

    What will be the concentration of the less soluble compund when the more soluble one begins to precipitate?

    2. Relevant equations
    Ksp AgCl = [itex]1.82 x 10^{-10}[/itex]
    Ksp Ag2CrO4 = [itex]1.2 x 10^{-12}[/itex]

    3. The attempt at a solution

    So, by calculating for their molar solubilities, I would know which one would precipitate first (which one is more soluble or less soluble)

    (2x)^2 (x) = Ksp Ag2CrO4
    x=Molar solubility of Ag2CrO4 = 6.69 x 10^-5 M
    (x)(x) = Ksp AgCl
    x= Molar solubility of AgCl = 1.35 x 10^-5 M

    Thus, Ag2CrO4 is more soluble and AgCl is the less soluble compound.

    Now, how will I find the concentration of the less soluble compound when the more soluble one begins to precipitate?

    Please guide me. Here's my attempt for a solution.

    The more soluble compound, Ag2CrO4 will begin to precipitate at this Ag+ concentration

    [Ag+]^2 [CrO42-] = Ksp Ag2CrO4
    [Ag+] = sqrt( Ksp Ag2CrO4 / [CrO42-] ) = 3.46 x 10^-6 M

    Now, how would I find the concentration of AgCl in the solution? Again, here is my attempt:

    I think, I should substitute the Ag+ concentration at the formula

    [Ag+][Cl-] = Ksp AgCl

    , But, is the Cl- concentration that I will get equal to the concentration of AgCl in the solution? I'm very confused. :(. Anyway, here's an attempt:

    [Cl-] = Ksp AgCl / [Ag+] = 5.26 x 10^-5 M

    I mean, the answer CAN be plausible since the concentration seemingly decreased. Is it correct guys?

    I think what I am having problem with is that why would the concentration of AgCl be equal to the equilibrium conc of Cl- ?
    Last edited: Oct 20, 2013
  2. jcsd
  3. Oct 22, 2013 #2
    Anyone here who can help me? Our test didn't include anything like this (thank god), but anyway, I still feel the need to learn this kind of stuff. It looks simple but I still can't be certain about this.

    Please help.
  4. Oct 22, 2013 #3


    User Avatar

    Staff: Mentor

    Sorry, somehow I missed your post earlier.

    What you did is correct. I guess the wording of the problem is wrong - it is a classic question, asked every year everywhere, and it always asks about concentration of the ion (Cl- here) and not "compound". Compound is ambiguous in this context which is why you are confused.
  5. Oct 22, 2013 #4
    Oh. Truth be told, I was wacking my head finding a similar problem in text books & internet and like you said, it's concerned only on the final equilibrium concentration of an ion. Thanks.

    I guess it's time to suggest to our institute to revise their laboratory manual, or at least their self-assesment questions. It has become more of a confusion rather than a help.

    Again, thank you very much. :)
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