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

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Discussion Overview

The discussion revolves around the nature of buffer solutions, specifically the combinations of weak acids and their conjugate bases versus weak bases and their conjugate acids. Participants explore the implications of using strong conjugate acids or bases in these mixtures and the apparent contradictions in buffer preparation guidelines.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the validity of using a weak base (Cl-) with a strong conjugate acid (HCl) for buffer preparation, suggesting it contradicts traditional guidelines.
  • Another participant argues that a buffer must consist of a weak acid or base that is not "too weak," implying that there is a threshold for effectiveness.
  • A different viewpoint suggests that buffers can be created from weak acids or bases along with their corresponding salts, challenging the strict definitions of weak and strong in this context.
  • Concerns are raised about the classification of weak acids and bases, particularly regarding their conjugates, with some participants noting that both can be weak despite the strength of their counterparts.
  • One participant highlights the relationship between pKa and pKb, indicating that a weak acid can have a weak conjugate base, which complicates the understanding of buffer solutions.
  • Another participant emphasizes the difference between pH and pKa, arguing that confusion between the two can lead to misunderstandings about buffer solutions.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and requirements for effective buffer solutions. There is no consensus on whether weak acids and bases must always have strong conjugates, and the discussion remains unresolved regarding the implications of using weak versus strong components in buffers.

Contextual Notes

Participants note limitations in understanding buffer solutions based on the definitions of weak and strong acids and bases, as well as the relationships between pKa and pKb. The discussion also reflects uncertainty about the practical implications of these classifications in buffer preparation.

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

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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?
 
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?
 
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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.
 
sci-doo said:
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.

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methods
 
Borek said:
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.
 
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.

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methods
 

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