Carboxylic Acids

Question

If a formic acid in formate buffer is 0.023 M in formic acid and has a pH of 4.07, what is the molar concentration of the formate ion? The pKa of formic acid is 3.75.

Answer 1

Formic acid = HCOOH

Formate = HCOO^-

Henderson Hasselbalch equation: pH = pKa + log [HCOO^-] / [HCOOH]

Solve for [HCOO^-]

4.07 = 3.75 + log [HCOO^-] / 0.023

0.32 = log [HCOO^-] / 0.023

2.089 = [HCOO^-] / 0.023

[HCOO^-] = 0.048 M

Answer 2