1. Chapter 15
Applied equilibrium

2. The calculations are the same as last chapter.
The Common Ion Effect
When the salt (with the anion of a weak acid) is added to a solution of the acid,
it stresses the equilibrium away from the dissociation of the acid and lowers the percent dissociation of the acid.
“common ion” means an equilbrium in which a WA or WB solution contains an ion that is the same as that in the acid or base
The same principle applies to salts with the cation of a weak base added to a solution of the base.
The calculations are the same as last chapter.

3. Solving Common Ion Effect Problems
1. Identify the major species present – don’t forget water is also present. 2. Write down the dissociation reaction (of either the weak acid or the weak base). 3. Set up the ICE table – but in this case, the initial concentration of the conjugate will be present from the salt. 4. Use Ka (if it is a weak acid) or Kb (if it is a weak base) to calculate the equilibrium concentrations.

4. Calculate the pH of a 0.15 M solution of HF (Ka = 7.2 x 10-4).
Sample Problem
Calculate the pH of a 0.15 M solution of HF (Ka = 7.2 x 10-4).
This solution does NOT contain a common ion.
pH = 1.98
HF ⇌ H+ F- I
0.15M C -x +x E
0.15-x x

5. Then calculate the pH when that same solution contains 0.015 M NaF.
Sample Problem
Then calculate the pH when that same solution contains M NaF.
This solution contains the common ion, F-
pH = 2.14
HF ⇌ H+ F- I
0.15 M
0.015 M C -x +x E
0.15-x x
0.015+x