Understanding dissociation constants

[sodium.dioxid]

Ka = [H+][A-]/[HA]

Is the term "[HA]" referring to its initial concentration or its equilibrium concentration?

 

[Borek]

All concentrations are equilibrium concentrations. Initial concentration of acid (sometimes called formal or analytical) is [HA]+[A^-].

 

[sodium.dioxid]

OK, another question: why is it necessary to have both [H+] and [A-] on the numerator? It seems like we could have done just as fine using the definition "Ka=[A-]/[HA]" or "Ka=[H+]/[HA]" instead of "Ka=[H+][A-]/[HA]". Is this conventional or am I missing something?

 

[Borek]

As both H⁺ and A^- are products of the reaction changing concentration of either one shifts the equilibrium, so you need both.

 

[LudusRex]

The term "[HA]" refers to the equilibrium concentration of the acid. This is because the dissociation constant, Ka, is a measure of the equilibrium between the dissociated form ([H+][A-]) and the undissociated form ([HA]) of the acid. In other words, it tells us how much of the acid has dissociated into its constituent ions at equilibrium. Therefore, the concentration of [HA] in the dissociation constant equation refers to the equilibrium concentration of the acid.