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

Answer:

Ka = 0.16

Explanation:

HClO₄ is a strong acid and it will dissociate completely to give hydrogen ion and the conjugate base.

The pH of HClO₄ will be calculated as:

pH = -log[H⁺]

pH = -log[0.04] = 1.4

The trichloroacetic acid is a weak acid and it will not dissociate completely.

The dissociation will be:

                $CCl_{3}COOH---> CCl_{3}COO^{-} + H^{+}$

Initial                     0.05                       0                         0

Change                -x                          +x                           +x

Equilibrium       0.05-x                     x                              x

Ka = $\frac{[H^{+}][ClO_{4}^{-}]}{[HClO_{4}]}$

As pH of the acid = 1.4 it means the concentration of hydrogen ion is 0.04.

Therefore

x = 0.04

Putting values

Ka = $\frac{(0.04)(0.04)}{0.05-0.04}=0.16$