Buffer Solutions: Weak Acid & Conjugate Base or Vice Versa?

[sci-doo]

Buffer solutions are supposed to be made of weak acid and its conjugate base or weak base and its conjugate acid. BUT: weak base has a strong conjugate acid and vice versa.

So I could make a buffer solution from Cl^- ions (really weak base) and HCl (conjugate acid). But that's the same as making buffer from HCl (strong acid) and Cl^- (conjugate base) which against the rule we had in the first place, isn't it. This contradiction makes understanding buffer solutions quite hard. Please explain!

 

[Borek]

Cl^- is so weak its of no use. To prepare buffer you have to use acid (base) that is weak, but not too weak.

--

 

[sci-doo]

But still, if you make a buffer solution from weak acid or base (with corresponging strong conjugates), its the same thing as making it from strong acid or base (with weak corresponding conjugates), isn't it?

So instead of making buffer solutions from weak acid and strong (conjugate) base or weak base and strong (conjugate) acid one should make it from not-weak-not-strong acid/base and not-weak-not-strong conjugate base/acid?

 

[sci-doo]

Like in this http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html" where buffer solutions are explained it is said:

Ammonia is a weak base, and the position of this equilibrium will be well to the left

but later:

The ammonium ion is weakly acidic, and so some of the hydrogen ions will be released again.

So there you have it, weak base that has weak conjugate acid? How can it be?

 

[symbolipoint]

Think about buffers from a different viewpoint, equivalent to what the instructional literature says, but worded from a different viewpoint:

Weak acid plus a salt of the weak acid;
OR
Weak base plus a salt of the weak base.

Either of those can be buffers.

 

[Borek]

weak base that has weak conjugate acid? How can it be?

In water

pKa + pKb = pKw

So weak acid with pKa 4 has a conjugate base with pKb 10. While obviously conjugate acid is much stronger than the conjugate base, they are still both classified as weak.

--
methods

 

[sci-doo]

In water

pKa + pKb = pKw

So weak acid with pKa 4 has a conjugate base with pKb 10. While obviously conjugate acid is much stronger than the conjugate base, they are still both classified as weak.

Ok, let's get back to the HCl. I don't know the pH of Cl^- but like you said its really weak and thus i guess quite neutral (7). Shouldn't the conjugate acid hydrogen chloride be pKa = pKw - pKb = 14-7=7 then neutral too (in aq). Don't make any sense.

I've always thought that weak acid must have strong conjugate base. Like CH4 is extremely weak acid but CH3 extremely strong base. But when there are weak bases having weak conjugate acids I must admit I don't get it.

 

[Borek]

pH and pKa are two different things, you are mixing them which suggests you have no idea what you are talking about. Check my pH calculation lectures.

pKa of hydrochloric acid is something like -4, which makes pKb of Cl^- close to 18. So very strong acid has very weak conjugate base - that's exactly as expected.

--
methods