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.