Explaining Resistance in pH of Buffer: Strong vs Weak Acids

[Samurai33]

could the equation - pH=pKa+log([A]/[HA]) - be applied for the case of strong acid? Although the percentage of dissociation of strong acid/base is very high, there still exists a equilibrium, right?

secondly, if i tried to explain the resistance in pH of buffer *in terms of* the above equation, then the pH of a buffer will depend only on the ratio of [A]/[HA]; then for both strong/weak acid with its salt, if i add H+ to each solution, the result will be: [A] decrease; [HA] increase (both cases). then how can i explain the difference that weak acid buffer has the ability to resist change in pH?

Please help me to solve this query, thanks a lot!

 

[Borek]

First question - Henderson-Hasselbalch equation works for strong acids too, although it can't be used for pH calculation, at least not in the same way as in the case of weak acids.

Second question is in fact about buffer capacity. Strong acids resist pH changes as well, although in slightly different way. In the case of strong acid pH changes only slightly in the case of relatively concentrated solutions, as neutralizing even 10% of acid doesn't change pH much. In the case of weak acids pH changes only slightly because weak acids are in a way inert - they almost don't dissociate on their own. Thus concentration of A^- and HA can be easily forced on the solution, and as long as they don't change much, pH doesn't change as well.

 

[Samurai33]

First question - Henderson-Hasselbalch equation works for strong acids too, although it can't be used for pH calculation, at least not in the same way as in the case of weak acids.

To be more specific, it cannot be used for calculating pH of buffer with strong acid and its salt??

 

[Borek]

See the page mentioned (part about dichloroacetic acid).

 

[GCT]

To be more specific, it cannot be used for calculating pH of buffer with strong acid and its salt??

You don't need to use the H-H equation, also, strong acids/bases are not employed for making standard buffers.

 

[Borek]

Ystrong acids/bases are not employed for making standard buffers.

How are you going to prepare pH = 1 buffer solution?