To determine the ratio of [a–]/[ha] of a solution of formic acid, we use the Henderson–Hasselbalch equation which relates the pH, pKa and the concentration of the ions in the solution. It is expressed as:
pH = pkA + log [a–]/[ha]
Substituting the values given,
3.75 = 3.75 + log [a–]/[ha]
0 = log [a–]/[ha]
[a–]/[ha] = 1
Therefore, the concentration of the CHOO- ion and the formic acid are equal.