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Mfouts
#1 Posted : Thursday, July 05, 2012 2:09:24 AM
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Hey,

So i'm getting a little confused with Ksp. Take for example: Ca(OH)2 --> Ca2+ + 2OH-

The Ksp equation is: Ksp = [Ca2+][OH-]^2

But, when you want to find the Ksp, you do the following: Ksp = [4x^3]

Based on other equilibrium constants you don't add the coefficient into the question and then square it, your simply put it to the power. So, why is this so for Ksp? If so, what other K function are like this?

Thanks
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lauradodge
#2 Posted : Thursday, July 05, 2012 7:35:32 PM
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Hello!

When writing a general formula for Ksp (as in examples where you are given the value for Ksp and you are asked to find the concentration of one of the ions in a certain solution) you would substitute "x" in for the number of moles of that ion. So the Ksp example you have for calcium hydroxide would become Ksp = [x][2x]^2 where the first x represents the number of moles of calcium released from dissolving Ca(OH)2 and 2x represents the number of moles of hydroxide released (this gets a coefficient 2 because there are two moles of hydroxide for every one mole of calcium).

When you simplify that formula, you need to square everything inside of the squared bracket (so you get 2^2 times x^2) which gives you 4x^2. Multiplying that by the x in the first set of brackets, you end up with Ksp = 4x^3

So ultimately it seems that the thing confusing you was the substitution of "2x" for the moles of hydroxide. Just remember that you are using "x" to represent moles of one of the species, and need to adjust it according to the stoichiometric relationship between all species involved.

I hope that was clear, please let me know if you need more clarification!

-Laura
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