Strong Acid and Weak Acid Solution pH

Acids dissociate and release H₃O⁺ (H⁺) ions in the aqueous state. But dissociation differ according to the acid. If acid is a strong acid, dissociation is complete. But, weak acids dissociate partially in the water and give less amount of H₃O⁺ ions to the water than strong acids.

In this tutorial, we will learn how an aqueous mixture of strong acid and a weak acid behave. And then calculate concentration of H₃O⁺ and pH of the solution.

First, we study dissociation of strong and weak acids and how weak acid dissociation is affected by a strong acid.

Dissociation of strong acid

Strong acid dissociates completely in the water. HCl, H₂SO₄ and HNO₃ are examples to strong acids. If we denote strong acid as HA and write the dissociation of it.

HA + H₂O → H₃O⁺ + A^-

• 0.1 mol dm^-3 HA will give 0.1 mol dm^-3 H₃O⁺ concentration.
• If we use 0.01 mol dm^-3 HA, it will give 0.01 mol dm^-3 H₃O⁺ concentration.

Dissociation of weak acid

Weak acid dissociates partially and reversibly in the water. Acetic acid (CH₃COOH), nitrous acid (HNO₂) and sulfurous acid (H₂SO₃ are examples to weak acids. If we denote weak acid as HB and write the dissociation of it.

HB + H₂O ⇌ H₃O⁺ + B^-

Concentration of H₃O⁺ is less than weak acid concentration

• 0.1 mol dm^-3 HB will give less than 0.1 mol dm^-3 H₃O⁺ concentration. It may be 0.0001 mol dm^-3 or 0.0005 mol dm^-3.
• If we use 0.01 mol dm^-3 HB, it will give 0.00004 mol dm^-3 H₃O⁺ concentration.

Le Chatelier's principles to weak acid dissociation

• Usually H₃O⁺ concentration of weak acid is low.
• If we add strong acid to weak acid solution, it will increase H₃O⁺ concentration in very large amount.
• Now equilibrium of weak acid try to keep equilibrium constant unchanged. So concentrations of H₃O⁺ and B^- should be decreased.
• So, dissociation of weak acid should decrease furthermore. That means, equilibrium point shifts to the left.

There is acidic mixture which contains 0.1 mol dm^-3 HCl and 0.1 mol dm^-3 CH₃COOH. Calculate followings.

1. H₃O⁺ concentration in the solution
2. CH₃COO^- concentration in the solution
3. pH in the solution

at 25⁰C dissociation constant (K_a) of CH₃COOH acid is 1.8 *10^-5 mol dm^-3

Assumptions

1. dissociation of water is negligible. Therefore H₃O⁺ from water is negligible.

Solution receives H₃O⁺ from HCl and CH₃COOH. When H₃O⁺ concentration increases, equilibrium point of dissociation of CH₃COOH goes left (according to the Le Chatelier's principle). Therefore dissociation of CH₃COOH is reduced.

Therefore concentration of H₃O⁺ is approximately equals to the H₃O⁺ which received from dissociation of HCl.

[ H₃O⁺ ] = [ H₃O⁺] from HCl

Now we can make two tables separately for strong acid and weak acid.

Then, we can write all equilibrium concentrations of each species.

• [CH₃COOH_(aq)] = 0.1-x
• [CH₃COO^-_(aq)] = x
• [H₃O⁺_(aq)] = 0.1+x

Apply equilibrium constant for equilibrium of ethanoic acid.

Now, we know the dissociated concentration. So we can find total H₃O⁺ concentration.

total H₃O⁺ concentration = H₃O⁺ concentration from HCl + H₃O⁺ concentration from CH₃COOH