Question

when a solution of benzoic acid was titrated with NIOS the pH of the solution when half the acid neutralize was 4.2 dissociation constant of the acid is

Answer 1

According to Henderson-Hasselbach equation, pH and the dissociation constant of the weak acid are related as:푝퐻=푝퐾푎+log([퐴−][퐻퐴])During the titration, when half of the acid is neutralized, the ratio [퐴−][퐻퐴]is equal to 1.This point in the titration is called half-equivalence point.Then, as the logarithm of 1 is zero, the pH at the half-equivalence point is equal to the pKa.Thus, pKaof weak acid is 4.2