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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: John123 on December 17, 2014, 06:29:36 AM

Title: Finding pH at second equivalence point
Post by: John123 on December 17, 2014, 06:29:36 AM

(I think) I know how to find the pH at the first equivalence point with pH=(pKa2 + Pka1)/2, but how do you find the pH at the second equivalence point? and the pH after that point?

-I'm only given Ka1 and Ka2 by the way, no Ka3 is provided.
-It is a weak base - strong acid titration
-You're not given how much titrant is added to reach the second end point

Title: Re: Finding pH at second equivalence point
Post by: Borek on December 17, 2014, 08:17:21 AM

At the second end point pH is typically that of a A^2- (a weak base) solution. If the pKa values are distant enough, you can safely ignore one of them. However, you can't ignore the concentration. Later it is mostly just an excess strong base being diluted.