[Big-Daddy]

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

Why was the charge balance dropped from the set of equations to be used in the above pH treatment of an amphiprotic salt's anion hydrolysis?

Nothing is being held constant by an unknown means, so we should know the identity of and be able to include all of the ions in the system, so why is charge balance not being used?

[Borek]

It is an approximate treatment, not an exact one, so the choice of equations is somewhat arbitrary.

[Big-Daddy]

Ok thanks. It also occurs to me that we are actually missing an ion, HA- must have been introduced to solution with some positive counter-ion, whose identity we don't know, therefore (since we can't include that ion in the charge balance) we have to drop the charge balance