Calculating pH Change in Buffer After Adding HCl

[coolia]

If you add HCl to a buffer solution of HF/NaF, the F- will react with H+ to produce HF. THe new pH can be calculated with the henderson-hasselbach equation. My question is, if F- decreases and HF increases, why and how does [H+] change?

HF <> F- + H+

Thank you.

 

[symbolipoint]

Essentially, the H+ from the strong acid HCl will increase the formal concentration of HF. Some of the salt NaF will be converted to HF...
... but let me be picky for a moment.

Actually, another point of view for hydrofluoric acid is the formula HFFH, a diprotic acid. The first dissociation is HFFH <----> H+ + FFH- ( the whole anion).
The second one is FFH- <----> H+ + FF-2

Please excuse the less than perfect text formatting.

 

[coolia]

I'm sorry, I don't understand how that answers the question. The use of HF/NaF is arbitrary any buffer will do.

 

[Borek]

If you add HCl to a buffer solution of HF/NaF, the F- will react with H+ to produce HF. THe new pH can be calculated with the henderson-hasselbach equation. My question is, if F- decreases and HF increases, why and how does [H+] change?

pH changes because you are adding acid - I hope that's clear and doesn't need further explanation.

Now, in the presence of conjugate base of weak acid, added H⁺ protonates the base, creating the acid (F^- + H⁺ <-> HF). If acid is weak, base is strong, so the reaction goes (almost) to completion. That in turn means that we can use stoichiometry of that reaction to calculate concentrations of acid and conjugate base using just stoichiometry, and put these values into HH equation.

But - and that's very important but - it doesn't meant every system and always behaves exactly this way. Too weak acid, too strong acid, too high dilution - and assumption that you can stoichiometry stops to work.

See Henderson-Hasselbalch equation and last paragraph on Buffer Maker pH engine page for examples of cases in which this approach doesn't work.

 

[coolia]

You are saying the pH changes by adding acid, but H+ reacts with the conjugate base so there no longer is a pH change. So then how does pH change?

 

[Borek]

What I mean is that if you add acid or base to the solution you disturb the equilibrium, so the pH must change. Presence of the buffer may make the change smaller, but it can't stop it.