1. Buffering Capacity Addition of STRONG Acids or Bases What is buffering capacity? What changes to pH occur with addition of strong acids or strong base?

2. Buffer Characteristics (Qualitative) 1. Buffer Capacity – amount of acid or base the buffer sln can neutralize before pH changes 2. Effective pH Range – numeric range over which the buffer acts effectively – Work to resist pH change best when pH = pK a – Use buffers whose acidic form pH ~ pK a

3. What is buffering capacity? Goal of a buffer is to keep the pH of a solution within a narrow range. While the ratio of [A - ]/[HA] influences the pH of a solution, the actual concentrations of A - and HA influence the effectiveness of a buffer. The more A - and HA molecules available, the less of an effect addition of a strong acid or base will have on the pH of a system.

4. Effect of Strong Acid or Base on Buffer Assume that all of strong is used up in rxns Strong completely dissociates, all H + or OH - Open text to pg. 711

5. Practice Exercise Calculate the pH of a solution that results when 0.10 mol gaseous HCl is added to 1 L of a buffered solution containing 0.25 M NH 3 (K b = 1.8 x 10 -5 ) and 0.40 M NH 4 Cl. LIST THINK STOICHIOMETRY H-H Eqn or [H +] eqn

6. Be a Chemist! Choose a Buffer! Optimal buffering occurs when [HA] = [A-] So, when choosing a buffer, you want [A-]/[HA] to be close to 1 The pK a of the weak acid to be used in a buffer should be as close as possible to the desired pH. Suppose you need a buffered solution with a pH of 4 Best choice for of a weak acid is one with pK a = 4 or K a = 1 x 10 -4