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Homework Help: Ksp and Q precipitate, Why does volume of water matter

  1. Nov 19, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    So heres my question again, why do you need to include 40L of water in the total volume? It's not like AgCl is going to dissolve in water right? AgCl has low solubility in water. I mean if it was like NaCl then you should include the water because it dissolved, but if it was something that isnt soluble in water at all, then why would you include the water?

    Lets say you did a reaction like this and it formed something that isnt solube in water at all, if you include the water in the math, the math will say no precipitate formed, but in reality there will be a precipitate formed.

    So for this reaction i'd say a precipitate will form but a small part of it would be dissolved
  2. jcsd
  3. Nov 19, 2016 #2


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    Staff: Mentor

    It is Q that defines what will precipitate and what will not. To calculate Q you need concentrations. How are you going to calculate concentrations without taking the final volume into account?
  4. Nov 19, 2016 #3
    Does it have anything to do with water being added in the beginning? So a precipitate never forms in the first place because the concentrations are too low, if it is then i guess i kind of understand it
  5. Nov 20, 2016 #4


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    Staff: Mentor

    Why would the situation be different if the water was added later? Why do you think AgCl would not dissolve? It may have low solubility, but it still follows Ksp. Plus, the final equilibrium doesn't depend on the path followed to get to the final state. It sometimes does, when reactions are irreversible, but that's not the case here.
  6. Nov 22, 2016 #5

    Dr Uma Sharma

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    Gold Member

    what is Ksp of AgCl ?
    it is the ionic product of [Ag+ ] and [Cl-] .These are the concentration of ions in solution and solution means solute (AgCl) and solvent( water here).It is not that AgCl is completely insoluble, its very low Ksp indicates that it is very less soluble.It means volume of water added do matter in precipitation of AgCl and cases will be there where Q<Ksp just because of excess water...like in your problem.
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