Question

A laboratory technician wants to determine the aspirin content of a headache pill by acid–base titration. Aspirin has a Ka of 3.0 × 10-4. The pill is dissolved in water to give a solution that is about 10-3 M and is then titrated with KOH solution. Find the pH at each of the following points, neglecting dilution effects:

(a) before titration begins.

(b) at the stoichiometric point.

(c) at the midpoint of the titration.

Answer 1

pka = - logka

    = -log(3*10^-4)

    = 3.52

(a) before titration begins.

   pH of weak acid= 1/2(pka-logC)

                  = 1/2(3.52-log10^-3)

                  = 3.26

(b) at the stoichiometric point.

   total aspirin converts into salt.

neglecting dilution effects , concentration of salt = 10^-3 M

   pH = 7+1/2(pka+logC)

       = 7+1/2(3.52+log10^-3)

       = 7.26

(c) at the midpoint of the titration.

in acid base titration , at the midpoint of the titration.

pH = pka = 3.52