Buffer/acid base titration question

In summary, when titrating a weak acid with a strong base, the pH is calculated like a buffer and the mixture consists of both the acid and its conjugate base. This does not act like a neutralization reaction with a strong acid and strong base because the Na+ is just a spectator ion and does not react with the A- to form a salt. As for the question about buffers, the acid and its conjugate base coexist in solution because they do not react with each other and instead form a weak acid and its conjugate base. This leads to no significant change in pH.
  • #1
LogicX
181
1
Lets say you titrate a weak acid, HA, with a strong base, NaOH.

Now, before the equivalence point, the pH is calculated like a buffer because HA + OH- -> A- + H2O. So you have a mixture of a an acid and its conjugate base. My question is, why doesn't this act like a neutralization reaction like a strong acid and strong base? In order to get OH- you have to dissociate NaOH, so you also have Na+ in solution. Why doesn't the Na+ react with the A- to form a salt like a strong acid and strong base would?

I'm also confused about buffers in general. You have an acid and its conjugate base... why don't they react? How can they coexist in solution?

Help, I seem to have a fundamental misunderstanding of acid-base chemistry!

EDIT:

WOW I'm dumb. I've been stressing over this for hours, thinking I didn't understand chemistry anymore. But obviously, the A- is in solution so its not going to form a precipitate because salts dissociate in water. Na+ is just a spectator ion.

Still not totally sure on the second question though.

Well, actually I guess they couldn't really react and change pH. If HA reacts with A- it would just form another A-...
 
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  • #2
You are right on both counts. There is a dissolved salt in the solution in both cases, and when weak acid reacts with its conjugate base products are weak acid and conjugate base - so nothing changes.
 
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FAQ: Buffer/acid base titration question

What is a buffer solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is made up of a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffers are important in maintaining the pH of biological systems and in chemical reactions.

How do I prepare a buffer solution?

To prepare a buffer solution, you need to mix a weak acid or base with its conjugate salt in a specific ratio. The ratio of the acid/base to the conjugate salt should be approximately 1:1. You can also use a buffer calculator to determine the exact amounts needed based on the desired pH and concentration.

What is the purpose of a titration in a buffer solution?

A titration is used to determine the concentration of an unknown acid or base in a solution. In a buffer solution, titration is used to determine the buffering capacity, which is the ability of the buffer to resist changes in pH when acid or base is added. This is important in understanding the effectiveness of a buffer solution.

How do you calculate the pH of a buffer solution?

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log ([A-]/[HA]), where pKa is the dissociation constant of the weak acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. Alternatively, you can use a buffer calculator to determine the pH.

Can a buffer solution be used to control the pH of any solution?

No, a buffer solution is only effective in controlling the pH of solutions that contain weak acids or bases. It cannot be used to control the pH of strong acids or bases, as they will completely dissociate and disrupt the equilibrium of the buffer system.

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