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.
strong acid weak acid solution pH - Le Chatelier's principle

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 (Ka) 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.

HCl and CH3COOH solution equilibrium

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.

strong acid and weak acid equilibrium concentrations

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.

acidic equilibrium constant for weak 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

pH of weak acid and strong acid solution



Questions


If strong acid concntration is too low, how we caalculate pH value?

If strong acid concentration is very lower than weak acid concentration, we cannot neglect the dissociation of weak acid. As an example, concentration of CH3COOH is 0.1 mol dm-3 and HCl concentration is 0.0001 mol dm-3, we have to consider the dissociation of CH3COOH acid.



Give several examples for a mixture of strong acid and weak acid solution?