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Ksp and Kb doubts

  1. Sep 25, 2012 #1
    I'm having some doubts about Ksp, solubility, Kb...

    1) If a salt has a solubility of 1g/L, it means that a) there is 1g of the salt itself in aqueous state in the solution or b) there is 1g of salt(aq) + salt ions in solution?

    2) The elements of the 2A family makes moderate bases. That's because they are too little soluble in water. Anyway, if we DO NOT reach the saturation point, can we consider that all (or >95%) of the aqueous base is dissociated (I mean, do they have a great Kb)? For what other bases can we consider dissociation = 100%?

    3) I was solving an exercise in where you had AgNO3, NaCl, NaBr. When mixing the solutions, it will form AgCl and AgBr, Ksp = 1.8.10^-10 and 5.10^-13
    When I saw the resolution, the author says we have to see if AgCl will precipitate, as AgBr precipitates first. I didn't understand that well. Why will AgBr precipitate first than AgCl? I know AgBr has a smaller Kb, but don't both salts have to precipite together?

    []'s
    João
     
  2. jcsd
  3. Sep 26, 2012 #2

    Borek

    User Avatar

    Staff: Mentor

    I don't understand the difference, please elaborate.

    Yes, they are weakly soluble but quite strong. Not that they dissociate 100% (although that's a reasonable first level assumption).

    Alkali metals.

    Salt starts to precipitate after its product of concentrations gets greater than the Ksp. In your example if we assume

    [Br-]=[Cl-]=10-10

    and

    [Ag+]=10-2

    it is easy to calculate

    [Ag+][Cl-]=[Ag+][Br-]=10-12

    so AgBr will already precipitate, but AgCl not yet.
     
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