Common ion effect Ksp problems (Gen Chem)

[Versati1ity]

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I just finished going through Destroyer #82 in the Gen Chem section which deals with the common ion effect affecting the solubility of the salt. This is the second problem common ion problem I have done where I am asked to ignore the x that is involved in the common ion effect.

I am given the Ksp and concentrations which come out to this

Ksp
1.6 x 10^-5 = (x) (2x + 0.01)^2

He say's since the Ksp is small, we can neglect the 2x as compared to 0.01M.
So when can we NOT neglect the 2x? What are the guidelines? Or do we always neglect it?

Let me know if you need more info, I just don't want to post the entire problem as written in destroyer.

 

[Versati1ity]

Let me know if I'm on the right track here.

Let's say

KCl -> K + Cl

Ksp = [K] [Cl]

The Ksp is the solubility product constant. The Ksp expression tells you the solubility of K ions and Cl ions. If given a Ksp of say 1.6 x 10^-5 that tells you that the solubility of the two ions multiplied together is 1.6 x 10^-5, so individually their solubilities is very small. So in the example I gave above the (2x + 0.01) 0.01 is WAY bigger and more significant than what 2x would be seeing as the Ksp is very small so you can ignore it and make your calculation easier.

Could someone give me an example of a problem where you actually do have to take it into account? Thanks