Question

At equilibrium, a solution of formic acid at 25°C has a pH of 2.18. The pKa of formic acid, HCOOH, is 3.74. Which of the following is the initial concentration of the solution?

Not sure what to do my thoughts:

use henderson hasselback equation

pH = pKa + log [conj base/acid]

use the provided pH to find [H+]

use an ICE table

Im not really sure

------
from what i have done i have been able to work out all the parts of the henderson hassel back but i don't know how to find the initial concentration of the entire solution.

Answer 1

This is not a buffer problem; therefore, the HH equation is not applicable.
......HCOOH ==> H^+ + HCOO^-
I......Y........0......0
C......-x.......x......x
E.....Y-x.......x......x

You know pKa, convert that to Ka.
Ka = (H^+)(HCOO^-)/(HCOOH)
The problem gives you pH so you know (H^+) = (HCOO^-) = x.
Substitute that into the Ka expression and solve for Y

Answer 2

thank you! :)