ionic eqlb

let us consider the basicity of the acid as n

then in the case of polytropic acids there will be a total of n dissociations

H_nA => H⁺ + H_n-1A^-
H_n-1A^- => H⁺ + H_n-2A^2-
...
...
..

HA^1-n => H⁺ + A^-n

let us consider the first reaction

we will not get the same concentration of H_n-1A^-
as in H_nA because that is possible only when it dissociates completely

so there will be less concentration for the next reaction

this will go on decreasing

depending on the Pka values we will consider only those reactions whose Pka values are appreciable

and then calculate the pH or pOH etc.