Calculate the 𝐾a for the acid

Question

Enough of a monoprotic acid is dissolved in water to produce a 0.0114 M solution. The pH of the resulting solution is 2.66. Calculate the 𝐾a for the acid.

𝐾a=

Answer 1

Let the monoprotic acid be HA

HA <==> H⁺ + A^-

Ka = [H⁺][A^-] / [HA]

From the pH, we can determine the [H⁺] and [A^-]. We then plug this value into the Ka equation and solve.

[H⁺] = 1x10^-pH = 1x10^-2.66

[H⁺] = 2.19x10^-3 M

Ka = ( 2.19x10^-3)( 2.19x10^-3) / 0.0114 - 2.19x10^-3

Ka = 4.80x10^-6 / 9.21x10^-3

Ka = 5.21x10^-4