Therefore, for the dissociation equilibrium of any acid
the acid dissociation constant, Ka, is defined as
This equation can be rearranged to make [H+(aq)] the subject:
Two assumptions are required:
1 Every A− ion comes from the salt
Although this is not quite true, it is a close enough that the pH value we get from the final equation is very close to that found experimentally. It allows us to assume that
2 Every HA molecule remains undissociated
Again, despite being slightly inaccurate, this assumption creates the following useful equation
The equations in assumptions 1 and 2 allow us to replace [A−(aq)] with [salt] and [HA(aq)] with [acid] as follows.