There is relationship between weak acid dilution and pH value. It is obvious, when weak acid is diluted, decrease of concentration of weak acid. In this tutorial, we are going to learn what is the relationship of these variations.
Weak acids partially dissociate into H+ (or H3O+) and anion in the water. Acetic acid (CH3COOH), formic acid (HCOOH) are some examples for weak acids.
Distilled water is added to the acid for diluting.
With decrease of concentration of the weak acid ( decrease of H+ concentration), pH increases. We can see a relationship between pH value, when we dilute the acid by 10, 100, 1000 times.
There are 3 bottles which contain aqueous ethanoic (CH3COOH) acid solutions. All ethanoic acids have different concentrations (im mol dm-3) as 0.1, 0.01, 0.001. Calculate pH of those solutions. equilibrium constant (Ka) of CH3COOH = 1.8 * 10-5 mol dm-3
Equilibrium constant expression is applied to find H3O+ concentration. Equilibrium constant value is given.
See the assumptions made in the weak acid dissociation and calculating pH values.
Now we know the pH values of each solution. Concentration of CH3COOH and pH value of each solution are listed below.
When weak acid is diluted by 10 times, concentration of it is decreased by 10 times.
When weak acid is diluted by 10 times, pH value is increased by 0.5
When weak acid is diluted by 100 times, pH value is increased by 1
When weak acid is diluted by 1000 times, pH value is increased by 1.5