# Problem: 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.

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###### FREE Expert Solution

HClO→ strong acid

pH = 1.398 (same pH as CCl3CO2H)

Remember that weak acids partially dissociate in water and that acids donate H+ to the base (water in this case).

The dissociation of CCl3CO2H is as follows:

CCl3CO2H(aq) + H2O(l)  H3O+(aq) + CCl3CO2(aq)

ICE Table:

Ka expression:

$\overline{){{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}{\mathbf{=}}\frac{\mathbf{\left[}{\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{\mathbf{+}}\mathbf{\right]}\mathbf{\left[}{\mathbf{CCl}}_{\mathbf{3}}{{\mathbf{CO}}_{\mathbf{2}}}^{\mathbf{-}}\mathbf{\right]}}{\mathbf{\left[}{\mathbf{CCl}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\mathbf{\right]}}}$

Solving for [H3O+] from pH:

$\overline{){\left[}{{H}}_{{3}}{{O}}^{{+}}{\right]}{\mathbf{=}}{{\mathbf{10}}}^{\mathbf{-}\mathbf{pH}}}$

${\mathbf{\left[}}{{\mathbf{H}}}_{{\mathbf{3}}}{{\mathbf{O}}}^{{\mathbf{+}}}{\mathbf{\right]}}\mathbf{=}{\mathbf{10}}^{\mathbf{-}\mathbf{1}\mathbf{.}\mathbf{398}}$

[H3O+ = 0.040 M

###### Problem Details

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.