Common ion effect

[Jordwin]

For some reason I'm not piecing together mathematically how the common ion effect decreases molar solubility. Because the concentration added is usually a number lower than one, so when you divide the ksp by this number lower than one, it increases, so the molar solubility increases right?

I get the overall idea that molar solubility decreases with the common ion effect, just not this math. Thanks!

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[Illfavor]

Ksp=[A]
Ksp(Common Ion)=[A][B-x]

Molar solubility and Ksp are reduced. You're dividing by [A] here to determine the .

 

[Jordwin]

Ksp=[A]
Ksp(Common Ion)=[A][B-x]

Molar solubility and Ksp are reduced. You're dividing by [A] here to determine the .

Isn't the [A] less than one usually though, which would make B greater?

 

[Illfavor]

Isn't the [A] less than one usually though, which would make B greater?

No, because Ksp will always be lower via the CIE. If it was constant, then B would increase, but it is not.

 

[Eri3]

Solubility problems involving Ksp follow this rxn. AB (s) <-----> A+(aq) B-(aq) ( a solid breaking down into its ions in H2O )

Remember that H2O can only break down a certain amount of solid until it becomes saturated. Thus when you add a common Ion - usually added as a salt (ex. XB) - the salt will also dissociate into its ions X+(usually a spectator ion so it is ignored) and B- (the common ion). The common ion(B-) is increasing the Ionic concentration in water thus less solid(AB(s)) will dissociate b/c the solution will reach saturation(equilibrium) faster.