PH, Buffers, and Dilution question

[habman_6]

I understand the henderson-hasselbalch equation, and how the pH of a solution depends on the ratio of acid to its conjugate base. However, I don't understand how this concept can be related to solutions that are extremely diluted.

For example, let's say you have 0.00001M acid+c.base. According to Henderson-Hasselbalch, the pH will still = pKa, since the log ratio is equal to 1. But does this make sense logically? I mean with that little acid, there are so little H+ ions dissociated, so how can the pH possibly equal the pKa (especially if the pKa is very low for that specific acid)??

 

[symbolipoint]

The dissociation of water becomes much more important for dilute solutions of acids (or bases). The K value for the weak acid or base still holds (not sure how well it holds when solutions are so dilute, like less than 0.001 F). Try a forum search. The more general form for the equilibrium constant expression has been discussed. The topic occurs fairly frequently.

 

[mysqlpress]

Well, in fact, there exists equation for calculation.
if the acid is in very low concentration, the approximation towards the water cannot be neglected.
Hence, the pH of the solution would be -log(sqrt Kw) =7 around 298K