Calculate pH of Acetic Acid (CH3COOH) | Examples | Online Calculator

Acetic acid (CH3COOH) is a carboxylic acid and behave as a weak acid in aqueous solutions. That means, acetic acid solution contains very low H+ ion concentration compared to equilibrium acetic acid concentration. Therefore, pH value of acetic acid solution is greater than HCl acid at same concentration solutions.


CH3COOH(aq) + H2O(l) ⇌ CH3COO-(aq) + H3O+(aq)



In this tutorial, we will discuss following sections.

Content

  • Dissociation of acetic acid
  • Calculate pH of acetic acid by using Ka value
    • Table of defining concentrations of acetic acid., acetate ion, hydronium ions
    • Substitute concentrations for expression of dissociation constant (Ka) value for acetic acid
    • Assumptions made during the calculations
  • Online calculator to find pH values of acetic acid solutions
  • pH values of acetic acid solutions for given concentrations
  • pH values of acetic acid at different concentrations


Dissociation of acetic acid (CH3COOH) in aqueous solutions

Some Acetic acid molecules dissociates poorly in water to acetate ion and hydronium ions. This reaction is reversible and equilibrium point is shifted to the left side (In strong acids such as HCl, equilibrium point is shifted to right side). That means, concentration of acetic acid is very much higher than concentrations of acetate ion and H+ ions.



Calculate pH of acetic acid by using dissociation constant (Ka) value

After writing down the equation of dissociation reaction, expression of dissociation constant is written. Equilibrium concentrations of compounds and ions should be substituted to the terms in the equation. Equilibrium concentrations are determined as following by creating a table.



Table of defining concentrations of acetic acid., acetate ion, hydronium ions

acetic acid pH calculation

Substitute concentrations for expression of dissociation constant (Ka) value for acetic acid

  • As mentioned earlier, Acetic acid is a weak acid. Therefore, dissociated concentration (x) is very small compared to the initial concentration (0.1 mol dm-3) of acetic acid.
  • x <<<< 0.1 mol dm-3
  • Therefore, we make an assumption of equilibrium concentration of acetic acid is same as the initial concentration of acetic acid.
  • Then, we can substitute the equilibrium concentration of acetic acid (CH3COOH), acetate ion (CH3COO-)and hydronium ion (H3O+) to the equation.

Calculate pH of acetic acid by using Ka value

Now, we know the concentration of H+ (H3O+) ions.

  • x = [H3O+(aq)] = [CH3COO-(aq)] = 1.34 * 10-3 mol dm-3


Assumptions made during the calculations

H3O+ is given by water is neglected because dissociation of water is very low compared to the acetic acid dissociation.


Because H3O+ concentration is known now, pH value of acetic acid solution can be calculated.

  • pH = -log[H3O+(aq)]
  • pH = -log[1.34 * 10-3]
  • pH = 2.88


Online calculator to find pH values of acetic acid solutions

Ka value of acetic acid at 250C is taken as 1.8 * 10-5 mol dm-3. Just, you have to enter concentration of acetic acid to the calculator to calculate the pH.




Answer


Try: weak acid pH online calculator

pH values of acetic acid solutions for given concentrations

Concentration of acetic acid (mol dm-3) pH
0.1 2.87
0.05 3.02
0.01 3.37
0.005 3.52
0.001 3.87
1 2.37

When acetic acid solution is diluted by ten times, it's pH value is increased by 0.5. As an example, when 0.1 mol dm-3 acetic acid is diluted up to 0.01 mol dm-3, pH value is increased from 2.87 to 3.37.



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