TBR Henderson-Hasselbalch question

[txiao5]

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Hmm... TBR says "according to pH=pka+log (conj base)/(conj acid), addition of water has no effect on pH. Compounds are diluted, and thus less concentrated, but pH remains the same because the A- : HA ratio remains the same.

I get that mathematically. How is it that it's less concentrated... doesn't that mean it either got less basic or less acidic...
I think we're talking about a buffer here right? I get that I'd have to have significant amount of both conjugate acid and base. So I think they're saying that concentration of both will stay the same since they're immersed in same amount of solvent. However won't pKa pKb kick in and one side will eventually become more dominant?

Also I read a post that was a good question, one that the OP understood but I didn't. According to the equation, as conjugate acid increases, pH of buffer increases. I understood that if pH < pKa then most of the compound remain in protonated form. However if I dump HCl into water... I'd have very very low HCl, very low pH, and very high conjugate base amount (Cl-)... How is it that conjugate acid amount increase lead to pH increase then... I understood at one point but can't quite remember now...
Sorry for the long post but i've been sitting on this for an hour now 😛

 

[rfvbnmju]

There's still the same amount of acid/base molecules but it's just in a lot more water.

The idea of a buffer is as long as the ratio of acid to conj base is the same, you'll have the same pH via Henderson-Hasslebach eqn.

The different between having 1M weak acid and 1M conj base vs. 0.1M weak acid and 0.1M conj base is the buffering capacity or how much the buffer can absorb before having a drastic change in pH.

Buffering capacity is +/- 1 of pKa. Mathematically, you can have 10:1 or 1:10 of acid to conj base.

The 1M buffer can resist changes in pH better than the 0.1M and you can toss in more strong acid/strong base into 1M buffer and not see a drastic change in pH. The 0.1M soln will still resist changes in pH but once there's no more weak acid or conj base to absorb the strong acid/base you put in, the pH will shift drastically.