Calculating pH Change in Buffer After Adding HCl

In summary, when HCl is added to a buffer solution of HF/NaF, the F- ions react with H+ ions from the HCl to produce more HF. This changes the formal concentration of HF and affects the pH of the solution. The new pH can be calculated using the Henderson-Hasselbach equation, which takes into account the concentrations of acid and conjugate base. However, this approach may not work in all cases, such as when the acid is too weak or the solution is too diluted. Ultimately, the pH of the solution will still change when acid or base is added, even in the presence of a buffer.
  • #1
coolia
19
0
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.
 
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  • #2
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.
 
  • #3
I'm sorry, I don't understand how that answers the question. The use of HF/NaF is arbitrary any buffer will do.
 
  • #4
coolia said:
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.
 
  • #5
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?
 
  • #6
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.
 

1. How do I calculate the pH change in a buffer after adding HCl?

The pH change in a buffer after adding HCl can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]), where pKa is the acid dissociation constant of the buffer, [base] is the concentration of the base in the buffer, and [acid] is the concentration of the acid being added.

2. What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation is a mathematical expression used to calculate the pH of a buffer solution based on the concentrations of its conjugate acid and base components. It is expressed as pH = pKa + log([base]/[acid]).

3. Can I use the Henderson-Hasselbalch equation for any buffer solution?

Yes, the Henderson-Hasselbalch equation can be used for any buffer solution as long as it contains a weak acid and its conjugate base, and the concentrations of these components are known.

4. What is the significance of the acid dissociation constant (pKa) in the Henderson-Hasselbalch equation?

The acid dissociation constant, or pKa, is a measure of the strength of an acid. It is used in the Henderson-Hasselbalch equation to determine how much the pH of a buffer solution will change when an acid or base is added.

5. How accurate is the Henderson-Hasselbalch equation in predicting pH changes in a buffer after adding HCl?

The Henderson-Hasselbalch equation is a good approximation for predicting pH changes in a buffer solution after adding HCl, as long as the concentrations of the components are relatively small and the pH is within the range of the pKa value. However, it may not be accurate for very strong or concentrated acids or bases.

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