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 x 10−4. If the pill is dissolved in water to give a solution about 0.0050 M, what is the pH of this solution? (Neglect dilution effects.)

If the solution in the problem above is then titrated against KOH solution, what will be the pH at the stoichiometric point.

Answer 1

Ka = 3.0 x 10^-4

concentration = 0.0050 M

aspirin = HA

HA ---------------> H+   + A-

0.0050                   0         0

0.0050 - x              x          x

Ka = x^2 / 0.0050 - x

3.0 x 10^-4 = x^2 / 0.0050 - x

x = 1.08 x 10^-3

[H+] = 1.08 x 10^-3 M

pH = -log [1.08 x 10^-3 ]

pH = 3.00