Some student performed titrations on 50.0 mL of a Zn^2+ solution with a concentration of 1.00x10^-2 mol/l with an EDTA solution whose concentration was 2.00x10^-2 mol//L at pH = 10.0 in the presence of NH3 with a concentration of 0.10 mol/L. Calculate the pZn value after adding 25.0 mL of EDTA solution.
K(ZnY^2-) = 10^16.5; a(Zn^2+) = 1.8x10^-5 and a(Y4) = 0.355.