Question

Trichloroacetic acid (CCl3CO2H) is a corrosive acid that is used to precipitate proteins. The pH of a 0.050 M solution of trichloroacetic acid is the same as the pH of a 0.040 M HClO4 solution. Calculate Ka for trichloroacetic acid.

Answer 1

Let's call trichloroacetic acid HT and perchloric acid is HClO4.
.............HClO4 ==> H^+ + ClO4^-
initial.......0.04.....0......0
change......-0.04......0.04...0.04
equil..........0.......0.04....0.04

............HT ==> H^+ + T^-
initial.....0.05...0......0
change.....-x......x.......x
equil.....0.05-x...x.......x

Ka = (H^+)(T^-)/(HT)
Ka = (x)(x)/(0.050-x)
The problem tells us that (H^+) in HT is the same as (H^+) in HT; therefore,
x = 0.04 and substitute into Ka expression for HT.
(0.O4)(O.O4)/(0.05-0.04) = ?