[sinthreck]

What is the pH of a 0.10 M solution of substance X, given that X acts as both acid and base? (pK_a1=5.37 and pK_a2=11.2
ANS: 8.285

My understanding:

H₂A + H₂O  HA^-    + H₃O⁺

HA^- + H₂O    HA^2-   + H₃O⁺

They have given us the concentration of HA^-

from here, I'm not sure where to go!

[rabolisk]

The easy and quick answer in these situations is that pH = (pKa1 + pKa2)/2.

Here's a link that explains it better than I could.
http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

[sinthreck]

cheers, we didn't cover this in this semester (just as well by the looks of it).

I was just going through old exam papers...