1. Acid – Base Equilibria Buffer Solutions: –Question: Was the ICE Problem set up needed? –Answer: No. The assumption of x << [HA], [A - ] is valid for all “traditional” buffers –Traditional Buffer Weak acid (3 < pK a < 11) Ratio of weak acid to conjugate base in range 0.1 to 10 mM concentration range

2. Acid – Base Equilibria Buffer Solutions: –Since ICE not needed, can just use K a equation –K a = [H + ][A - ]/[HA] = [H + ][A - ] o /[HA] o (always valid) (valid for traditional buffer) –But log version more common –pH = pK a + log([A - ]/[HA]) –Also known as Henderson-Hasselbalch Equation

3. Acid – Base Equilibria Buffer Solutions: –Ways to make buffer solution: Mix weak acid and conjugate base Add strong base to weak acid (weak acid must be in excess) – this converts some of the weak acid to its conjugate base Add strong acid to weak base (weak base must be in excess) – this converts some of weak base to its conjugate acid

4. Acid – Base Equilibria Example Problems: –How many moles of hydroxyl ammonium chloride (HONH 3 + Cl - ) needs to be added to 500 mL of 0.020 M HONH 2 to obtain a buffer solution with a pH of 6.20? The pK a for HONH 3 + is 5.96. –What is the pH of a solution made from mixing 400 mL of 0.018 M CH 3 CO 2 H (pK a = 4.75) with 100 mL of 0.024 M NaOH?

5. Acid – Base Equilibria Last Example Problem: –How many mL of 0.0500 M NaOH should be added to 50.0 mL of 0.00850 M methyl ammonium chloride (CH 3 NH 3 + Cl - ) in order to make a buffer with a pH of 10.80? The pK a for CH 3 NH 3 + is 10.645.

6. Acid – Base Equilibria Additional Questions: –Which of the following will result in a traditional buffer? 0.00100 M HNO 3 + 0.00200 M NaNO 3 0.010 M NH 4 Cl + 0.003 M NH 3 1.0 x 10 -5 M CH 3 CO 2 H + 0.010 M NaCH 3 CO 2 0.0020 M HCl + 0.010 M CH 3 CO 2 H 0.0020 M HCl + 0.010 M NaCH 3 CO 2