Acetic acid (CH3COOH) is a carboxylic acid and behave as a weak acid in aqueous solutions. That means, acetic acid solution contains very low H+ ion concentration compared to equilibrium acetic acid concentration. Therefore, pH value of acetic acid solution is greater than HCl acid at same concentration solutions.
In this tutorial, we will discuss following sections.
Some Acetic acid molecules dissociates poorly in water to acetate ion and hydronium ions. This reaction is reversible and equilibrium point is shifted to the left side (In strong acids such as HCl, equilibrium point is shifted to right side). That means, concentration of acetic acid is very much higher than concentrations of acetate ion 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.
Now, we know the concentration of H+ (H3O+) ions.
H3O+ is given by water is neglected because dissociation of water is very low compared to the acetic acid dissociation.
Because H3O+ concentration is known now, pH value of acetic acid solution can be calculated.
Ka value of acetic acid at 250C is taken as 1.8 * 10-5 mol dm-3. Just, you have to enter concentration of acetic acid to the calculator to calculate the pH.
|Concentration of acetic acid (mol dm-3)||pH|
When acetic acid solution is diluted by ten times, it's pH value is increased by 0.5. As an example, when 0.1 mol dm-3 acetic acid is diluted up to 0.01 mol dm-3, pH value is increased from 2.87 to 3.37.