Solving Ksp problems/Gen Chem

[calicarbocation]

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If you are given a chemical equation and given the Ksp value and told to solve for the concentrations of the dissociated ions then how do you know if you're supposed to use the coefficients when solving?
for example....
AB2 ---> A + 2B

Ksp = (A)(2B)^2 ....OR......Ksp= (A)(B)^2

which is correct!?!
I've encountered both forms in answer keys and now I'm confused

 

[UndergradGuy7]

If you are given a chemical equation and given the Ksp value and told to solve for the concentrations of the dissociated ions then how do you know if you're supposed to use the coefficients when solving?
for example....
AB2 ---> A + 2B

Ksp = (A)(2B)^2 ....OR......Ksp= (A)(B)^2

which is correct!?!
I've encountered both forms in answer keys and now I'm confused

I think you mean

If AB2 ---> A + 2B
then we can say we have x of A and 2x of B.
so Ksp = (x)(2x)^2 which simplifies to (x)(4x^2) = 4x^3

 

[Baylor2011]

I think you're getting confused between a Ksp expression and actually setting up the ice chart and solving to get molar solubility.

If it asks just for the Ksp expression...it'd be (a)(b)^2

If it asks you to calculate the molar solubility..you have to set up an ice chart

AB2 ----> A + 2B
I ---------0----0
C---------+x--+2x
E----------x----2x

Then plug it in the Ksp expression

Ksp = (x)(2x)^2
Ksp = 4x^3