Formic acid (HCOOH) is a weak acid and it belongs to the carboxylic acid compounds in organic chemistry. That means, aqueous formic acid solution contains very low H+ ion concentration compared to the undissociated formic acid concentration. Therefore, pH value of formic acid solution is greater than HCl acid at same concentration solutions.
In this tutorial, we will discuss following sections.
Formic acid dissociates poorly in water to formate ion and hydronium ion. This reaction is reversible and generally equilibrium point is shifted to the left side (In strong acids such as HCl, equilibrium point almost is shifted to right side). That means, concentration of formic acid is very much higher than concentrations of formate ions and H+ ions.
After writing down the equation of dissociation reaction, expression of dissociation constant is written. Equilibrium concentrations of compounds and ions should be substituted to the terms in the equation. Equilibrium concentrations are determined as following by creating a table.
The equation for dissociation constant can be written as below.
Assume that we are going to find the pH of 0.1 mol dm-3 formic acid solution.
As earlier mentioned, formic acid is a weak acid. Therefore, dissociated concentration is very small compared to the initial concentration of formic acid.
Therefore, we make an assumption of equilibrium concentration of formic acid is same as the initial concentration of formic acid. Then, we can substitute the equilibrium concentration of formic acid (HCOOH), formate ion (HCOO-)and hydronium ion (H3O+) to the equation as following.
Now, we know the concentration of H+ (H3O+) ions.
H3O+ concentration given by water is neglected because dissociation of water is much lower compared to the formic acid dissociation.
Because H3O+ concentration is known now, pH value of formic acid solution can be calculated.
Ka value of formic acid at 250C is taken as 1.8 * 10-4 mol dm-3.
|Concentration of formic acid (mol dm-3)||pH|
When formic acid solution is diluted by ten times, it's pH value is increased by 0.5. As an example, 0.1 mol dm-3 formic acid is diluted upto 0.01 mol dm-3, pH value is increased from 2.37 to 2.87.