Acids and conjugate bases

[BurningKrome]

So, another basic question (no pun intended). Acids and their conjugate bases.

 

The definition of a conjugate base is the resulting base after the hydrogen(s) have been removed from the acid. I.e. acid CH3COOH has a conjugate base of CH3COO-.

 

 

I also understand that, by definition, the stronger an acid the weaker the conj base. The fact that the conjugate base is weak is WHY the acid is strong because the base cannot hold onto the proton. I.E. H2SO4 (Very strong acid, Ka = 1.0 * 10^3) IS so strong specifically because HSO4- (Kb = 1x10^-17) is such a weak base that it can't hold onto the H+ in solution.

 

 

The fact that Kb = Kw / Ka (where Kw = 1x10^-14) means expressly that the larger the Ka (stronger) the smaller the Kb (weaker).

 

 

However, an axiom of acid/base (specifically buffer solutions) is that a weak acid has a weak conjugate base. This does not make sense to me based on the above. It seems more accurate to say that a "Moderate acid has a moderate base…whereas a weak acid has a strong base and a strong acid has a weak base".

 

 

Is this simply a matter of semantics…where a weak acid has a relatively weak base when compared to other bases and acids…or am I missing something more critical here?

Edited September 30, 2010 by BurningKrome

[Mr Skeptic]

I also understand that, by definition, the stronger an acid the weaker the conj base. The fact that the conjugate base is weak is WHY the acid is strong because the base cannot hold onto the proton. I.E. H2SO4 (Very strong acid, Ka = 1.0 * 10^3) IS so strong specifically because HSO4- (Kb = 1x10^-17) is such a weak base that it can't hold onto the H+ in solution.

 

The fact that Kb = Kw / Ka (where Kw = 1x10^-14) means expressly that the larger the Ka (stronger) the smaller the Kb (weaker).

 

I think the problem there is you misunderstand what a weak base is, it really hasn't been named properly. Strong and weak are simply indications of how thoroughly it dissociates. For example, HF is a weak acid but it is a nastier acid than HCl which is a strong acid. For a strong acid, its conjugate base is essentially inert.

[BurningKrome]

You may be right. I get that a strong base like HCl is strong because Cl- is basically not a base at all and it disassociates completely in water. NaOH is considered a strong base because it also completely dissassociates completely in water and the Na+ is basically non-acidic.

 

So, maybe my misunderstanding is this: My understanding of Kb (K-base) is the ability to which a base can accept electrons (Lewis) or RETRIEVE an H+ (Bronstead). Is this acurrate?