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 (K_a) 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.

Calculating concentrations of CH₃COOH solutions

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.

• 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.