Question

pH of a 1.0 M solution of formic acid (HCOOH), a weak organic acid found in red ants and responsible for the sting of their bite, is 1.88. What is the K value of the acid?

Answer 1

Answer:

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