Why do buffers require a salt?

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stester77s

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I am trying to understand why is the salt necessary in a buffer?

For example, in CH3COOH and CH3COONa, Does the salt just provide so much more CH3COO- ion than the acid alone would otherwise?

Secondly, what is the salt in the H2CO3 buffer system in the human body?

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Hmmm. I think the salt is just to hold the ion in place, and when it enters an aqueous solution (where the weak acid would be) it would break down into the constitute and the conjugate thus completing the buffer.
The cb = hco3-. Salt could be nahco3
 
CH3COOH is a weak acid. Its Ka is something like ## x 10^-5. Idk. When a respectable amount is added to water (1M, 0.1M), yes some of it will form H3O+ and leave behind the anion, but the majority of it will still be CH3COOH itself. That's why the you need to add more anion to make a nice balance of acid and conjugate base so pH=pKa.
This and the fact that an anion alone cannot be introduced into a solution. It's usually in combination with a neutral cation such as sodium (or for a cationic acid such as NH4+, a neutral anion like Cl-). When sodium ions dissociate into the solution they are neither acidic or basic which is great for buffers because then you only need to concern yourself with having an equal amount of acid and it's conjugate base -- without concern for side reactions.
 
Secondly, what is the salt in the H2CO3 buffer system in the human body?
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That's a complex question, but just realize that when our tissues give off CO2, it diffuses into RBC's and there it combines with H2O via the enzyme Carbonic Anhydrase to form H2CO3, a highly reactive acid that dissociates to bicarbonate and protons. This general reaction can occur in either direction depending on the CO2, H+, and O2 content in our blood and it occurs readily at any given instant. In this case you're not making a buffer solution and so there's no need to concern yourself with any salt.
 
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