Acid equilibrium constant confusion

[bentonj002]

I know that Ka = [H+][A-]/[HA]. By that equation Ka would also = [H+]^2/[HA] since when HA dissociates it dissociates into equal parts H+ and A- correct? I think I'm just over thinking the whole thing, but I would just like to make sure. Thank you.

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[Cawolf]

It would if it was dissolving in a solution of pure water.

But since you want to apply the mass expression to buffers and non-pure solutions it is best to see how common ions effects the acid dissociation.

So if you put the acid in a solution that already contained protons, you need to proper expression to calculate the dissociation.

 

[bentonj002]

Ok, that's what I thought. Thank you for clearing that up for me.