Trichloroacetic acid (HC2Cl3O2) is a corrosive acid that is used to precipitate proteins. The pH of a 0.072 M solution of trichloroacetic acid is the same as the pH of a 0.057 M HCl solution. Calculate Ka for trichloroacetic acid.

Explanation:

HCl is strong acid, hence 0.04M of acid

= 0.057 M of H+

pH = -log [H+]

= -log [0.057]

= 1.244

Let the dissociation of trichloroacetic acid be:

HClO4 --> H^+ + ClO4^-

HA --> H+ + A-

0.072 - x x x

[H+] = 0.057 M

[HA] = 0.072 - 0.057

= 0.015 M

[H+] = [A-] = 0.057

Ka = [H+][A-]/[HA]

= (0.057^2)/(0.015)

= 0.217

= 0.22.