Question

the pH at the equivalence point of a titration may differ from pH 7.0 due to _____________

Answer 1

So the direct answer is because of slat hydrolysis the pH at equivalence point will differ from 7.

let understood how it is possible.

In acid-base titration, slat is being formed. first, we will take an example of titration of a weak acid with a strong base, let's say we have taken weak acid acetic acid and strong base as NaOH. now if we start constantly adding the base in acid, what will happen? NaOH reacts with CH3COOH to form, CH3COONa salt.

CH3COONa + NaOH ----> CH3COONa + H2O

now this salt has a tendency to dissociate or hydrolyze in water to form acetic acid because the acetate ion came from a weak acid, so the conjugate ion is strong. but Na+ didn't hydrolize because it came from a strong base, the conjugate ion is weak.

CH3COONa + H2O <=> CH3COOH + OH-

So in the solution, there is a generation of OH- happened during the end point so as it is basic, the pH of the neutralization point is gets increased from 7.

let's take another example, titration of a weak base with a strong acid.

let us say the weak base is NH4OH and Strong acid is HCl

so, NH4OH + HCl -----> NH4Cl + H2O  

now the NH4Cl salt is hydrolyzed to form NH4OH and free H_+ in the system during neutralization.

as, NH4Cl + H2O <=> NH4OH + H+

so as the free H+ is generated, the pH of the neutralization point decreased from 7.

now for the case of strong acid and strong base neutralization, no salt hydrolysis occurs because both the conjugate ions came from the strong acid so both of them are weak.