# ksp question

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#### mgdent

##### Full Member
10+ Year Member
i was looking at one of the kaplan flash cards and it says the ksp for the reaction Fe(OH)3--> Fe3+ + 3 OH- is [Fe3+][OH-]^3 but shouldnt it be [Fe3+][3OH-]^3? Since it is MX3= 27x^4 it would have to be (x)(3x^3).

Well no,

they are right.. the Ksp they give u is right, because when they are asking for the Ksp its just the breakdown of the solution...

when you are figuring out the Ksp, then u plug in the (3x) and ^ 3 power...

i was looking at one of the kaplan flash cards and it says the ksp for the reaction Fe(OH)3--> Fe3+ + 3 OH- is [Fe3+][OH-]^3 but shouldnt it be [Fe3+][3OH-]^3? Since it is MX3= 27x^4 it would have to be (x)(3x^3).

Think about what 3x actually is. It is the concentration of OH- at equilibrium. So, 3x = [OH-]
Thus (3x)^3 = [OH-]^3

i was looking at one of the kaplan flash cards and it says the ksp for the reaction Fe(OH)3--> Fe3+ + 3 OH- is [Fe3+][OH-]^3 but shouldnt it be [Fe3+][3OH-]^3? Since it is MX3= 27x^4 it would have to be (x)(3x^3).

If you have the Ksp and you want to solve for x then you would use this equation. You normally use this equation for weak acids and bases. If you have a common ion effect question, you will use the common ion's concentration and whatever is left over. If you have a common ion like OH- = .1 M you will use [x][.1]^3 = Ksp

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If you have the Ksp and you want to solve for x then you would use this equation. You normally use this equation for weak acids and bases. If you have a common ion effect question, you will use the common ion's concentration and whatever is left over. If you have a common ion like OH- = .1 M you will use [x][.1]^3 = Ksp
So, do you always just simply ignore the coefficient (but keep the exponent the same) when it's an MX2, MX3, etc. salt?

I know its confusing, but there's two different types of situations. If you are given a concentration of the solid (reactant) and asked to find the concentration of a product than you use the coefficient (3x)^3. But if you are given the concentration of one of the ions and asked to find ksp or something than you don't use the coefficient, just the concentration given cubed ([OH-])^3

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