[hilbuby]

Consider the following titration curve for a 1.0 M solution of a diprotic acid, H₂As with NaOH.



What are the approximate pH values at points 1,2,3,4?

Here's what I already have.. the major species at point;
1. H₂As, HAs-
2. HAs-
3. HAs-, As²-
4. As²-

and the pH values at points;
1. 4.10
2. 7.95
3. 11.79
4. ?? This is the one I'm having problems with. I was told to use K_b values so I set up K_b = K_w/K_a, what do I do with that value(s) though?

THANKS SO MUCH!! 

[Borek]

You are correct so far.

At the second equialence point you have just a solution of a salt of weak acid. That means the best approach is to calculate pOH from Kb and concentration of conjugate base, then convert pOH to pH.

Argh, the question is stupid, stupid, stupid. See attached CORRECT titration curve. It has ONE inflection point.

http://www.titrations.info/titration-curve-calculation
http://www.chembuddy.com/?left=pH-calculation&right=titration-curves-calculation