# PH, Buffers, and Dilution question

1. Oct 12, 2007

### 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 dont understand how this concept can be related to solutions that are extremely diluted.

For example, lets 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)??

2. Oct 13, 2007

### 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.

3. Oct 28, 2007

### 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