The mass balance equation calculates the unknown concentration the dissociated particle (the anion). The equation is based of the matter conservation theorem, where the amount of what was put in to the solution must remain in the solution.
The equation for mass balance is:
DIS = IMC /(UDA*DP)
where
Notice: the above equation solves for one molar concentration of the unknown dissociated products.
Harris, 9th Edition. Pp.170