Which acid dissociation constant would I use?

Question

I'm trying to find the concentration of all the species present after a 0.10 M solution of oxalic acid is added to water. The problem is that there are two acid dissociation constants, one is 6.5x10^-2 and the other is 6.1x10^-5. Would I use one, the other, or both?

I got it, it's a polyprotic acid, thus it basically dissociates twice.

Well, I figured it out myself, so at this point, a good chemistry joke (or probably any other joke) is worth ten points.

Answer 1

Since the first Ka is about 1,000 times the second one, it will provide the majority of the H+ ions, and you can ignore the second one.

OK: what did the lost proton say to the electron? I'm positive I should be in the nucleus....;