Chapter 15 Applied equilibrium

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.

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.

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

Then calculate the pH when that same solution contains 0.015 M NaF. Sample Problem Then calculate the pH when that same solution contains 0.015 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