The pH of a 1.00 M solution of formic acid (HCOOH), a weak organic acid found in red ants and responsible for the sting of their bite, is 1.87. What is the Ka value of the acid?

Question

How would you also find the percent ionization of 1.00 M formic acid?

Answer 1

HCOOH(aq) ==> H⁺(aq) + HCOO^-(aq) ... dissociation of formic acid

Ka = [H⁺][HCOO^-]/[HCOOH]

From the pH, we can find the [H⁺] and [HCOO^-] and we are given the [HCOOH]. Thus, we can then find Ka.

pH = 1.87

[H⁺] = [HCOO^-] = 1x10^-1.87 = 1.35x10^-2 M

Ka = (1.35x10^-2)(1.35x10^-2) / 1.00 - x (and x is small so can ignore it in denominator)

Ka = 2.25x10^-4