Question

3. Explain why the pH at the equivalence point of the titration of vinegar with sodium hydroxide is greater than 7.00 while the equivalence point of the titration of Hydrochloric acid with sodium hydroxide is 7.00.

Answer 1

An acid- base titration is used to determine the unknown concentration of an acid or base by neutralising it with an acid or base of known concentration. That means neutralisation is the basis of titration. A pH indicator shows the equivalence point, which is the point at which the equivalent number of moles of a base have beed added to an acid.

Consider the titration between hydrochloric acid with NaOH.

HCl(aq) + NaOH(aq) $\rightarrow$ H2O(l) + NaCl(aq)

In strong acid- strong base titration the neutralisation result in a solution with pH 7.00. Note that this is true only for titrations of strong acid with strong base. Since HCl is a strong acid and NaOH is a strong base ( high values of Ka and Kb) both will fully dissociate and at the equivalence point, equal qmount of H+ and OH- will combine to form water along with a salt which is neutral. So we can say that since neither H+ nor OH- molecules remains in the solution, at equivalence point the pH always equal to 7.00.

But the weak acid will react with a strong base to form a basic ( pH>7) solution. Vinegar ( CH3COOH is the acid in vinegar) is a weak acid and NaOH is a strong base. At the equivalence point, all of the weak acid is neutralised and converted to its conjugate base. But the pH at the equivalence point is not equal to 7. This is due to the production of conjugate base during titration.The resulting solution is slightly basic.

CH3COOH + OH- $\rightarrow$ H2O + CH3COO-

During the titration, as the OH- react with the H+ from acetic acid, the acetate ion is formed. This conjugate base react with water to form a slightly basic solution.

CH3COO- + H2O  $\rightarrow$ CH3COOH + OH-