For the reaction,Hg2++2Cl−⇌HgCl2,K=1.65×1013concentration of Hg2+, at the equivalence point in the titration of 2.0 mmol of Hg2+ with CI- when final volume is 100 mL, is

Hg2+=2.0 millimoles in 100mL solution  =2×10−3mol0.1L =2×10−2M=0.02MSince, stability constant of HgCl2 is large (1 .65x1013), the value of [Hg2+] should be small.At the end point, Cl−=2Hg2+HgCl2=0.020−Hg2+≈0.02M ∴ HgCl2Hg2+Cl−2=1.65×1013 0.02Hg2+2Hg2+2=1.65×1013 ∴ 4Hg2+3=0.021.65×1013 ∴ Hg3+=6.172×106M