Formic acid has a pKa of 3.8.Calculate the pH of a solution if [Formic acid] = 0.020 M and [Formate] = 0.040 M.

Question

Quizplus · Accepted Answer

The equilibrium reaction for the dissociation of formic acid is:

HCOOH ⇌ H+ + HCOO-

The equilibrium constant expression for this reaction is:

Ka = [H+][HCOO-] / [HCOOH]

We can use the Henderson-Hasselbalch equation to relate Ka, the concentrations of the acid and its conjugate base, and the pH of the solution:

pH = pKa + log([HCOO-]/[HCOOH])

Substituting the given values we get:

pH = 3.8 + log(0.040/0.020) = 4.4

Therefore, the pH of the solution is 4.4, which corresponds to choice D.

Formic Acid Has a PKa of 3