Relationship of pH and diluting weak acid

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.

You should study dissociation of weak acid example before this example

What is a weak acid?

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.

Diluting the acid

Distilled water is added to the acid for diluting.

Relationship of pH and H+

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.

pH values of different concentrations of ethanoic acid solutions

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

Find pH of each solution

Equilibrium constant for ethanoic acids to find H3O+ concentration

Equilibrium constant expression is applied to find H3O+ concentration. Equilibrium constant value is given.

Equilibrium constant expression of CH3COOH

Calculating concentrations of CH3COOH solutions

See the assumptions made in the weak acid dissociation and calculating pH values.

solving equilibrium constant expression

Now we know the pH values of each solution. Concentration of CH3COOH and pH value of each solution are listed below.

  • 0.100 mol dm-3 : 2.87
  • 0.010 mol dm-3 : 3.37
  • 0.001 mol dm-3 : 3.87

pH versus weak acid concentration

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