The Common Ion Effect

Learning Goals

You will be able to predict the pH of an acidic solution when there is an ion already present in the solution that is common to an ion of the acid.


In a sense this section deals with another application of the Le Chatelier principle. A weak acid, HA, will ionize in a water solution to produce some hydronium ions. However if some A- ions were already in solution before the acid were added, the reaction would be shifted to the reactant side because of the increased amount of A- ions compared to a solution without additional A- ions. Consider the example of acetic acid in a solution without any additional acetate ions (solution X) and in a solution which originally contained some sodium acetate (solution Y). Solution Y would be shifted more to the left than solution X because of the additional acetate ions (product ions from the acetic acid perspective). Therefore solution Y would have a lower concentration of hydronium ions and therefore a higher pH than solution X.

Review Questions

  1. Calculate the pH of a 0.100-M solution of acetic acid.
  2. Calculate the pH of a 0.100-M solution of acetic acid when the water already had a 0.100-M concentration of acetate ions from adding sodium acetate.
  3. Compare the results of questions 1 and 2 and calculate how much concentration of a common ion would have to be present to increase the pH of an acetic acid solution by 50%.


Web Author: Dr. Leon L. Combs
Copyright 2000 by Dr. Leon L. Combs - ALL RIGHTS RESERVED