[dagr8est]

Hello, I did an experiment at school where I first titrated 0.1M HCl with 0.1M NaOH, and then 0.1M CH₃COOH with 0.1M NaOH.

The pH at the equivalence point of the weak acid titration was above 7.00 because some of the CH₃COO^- hydrolyses to form OH^- which makes the solution basic.

I also noticed that after the equivalence point, if the same amount of NaOH was added to both solutions, the pH of both solutions were the same.  My question is why are they the same?

My initial thought was that the OH^- produced by the hydrolysis is insignificant compared to the OH^- produced by the dissociation of NaOH.  Is that the reason?  Any help would be appreciated.

[tamim83]

I also noticed that after the equivalence point, if the same amount of NaOH was added to both solutions, the pH of both solutions were the same.  My question is why are they the same?

I am pretty sure that this was a coincidence.  The [OH-] concentration is signifigant enough for the weak acid titration to make the pH be around 8-ish.  One reason you experienced this is that adding even a drop of strong base past the equivalence point is enough to make the pH increase greatly, which is why you noticed the similar magnitude pH increase for both titrations. 

[dagr8est]

Hmm, I guess I didn't make myself clear enough.  The pH at the equivalence point for HCl is 7.00 and for CH₃COOH is 8.87.  However, every pH after that is the same for both acids.  One of the questions asks why the pH is exactly the same beyond the equivalence point for both titrations.

[Borek]

Hmm, I guess I didn't make myself clear enough.  The pH at the equivalence point for HCl is 7.00 and for CH₃COOH is 8.87.  However, every pH after that is the same for both acids.  One of the questions asks why the pH is exactly the same beyond the equivalence point for both titrations.

You are more or less right - amount of OH^- added after the end point is much larger than amount of OH^- produced from the CH₃COO^- hydrolysis. Note that there is a small alrea (somewhere between 100% and 100.1% titration) where the pH differs.