 Take out your purple sheet, 17.2 worksheet, and problem set 1  Questions on 17.2 worksheet?  Questions on problem set 1?

 The Henderson–Hasselbalch equation is generally good enough when the “x is small” approximation is applicable.  Generally, the “x is small” approximation will work when both of the following are true: a)The initial concentrations of acid and salt are not very dilute b)The K a is fairly small.  For most problems, this means that the initial acid and salt concentrations should be over 100 to 1000 times larger than the value of K a.

 Calculating the new pH after adding acid or base requires breaking the problem into two parts: 1.A stoichiometry calculation for the reaction of the added chemical with one of the ingredients of the buffer to reduce its initial concentration and increase the concentration of the other. Added acid reacts with the A − to make more HA Added base reacts with the HA to make more A − 2.An equilibrium calculation of [H 3 O + ] using the new initial values of [HA] and [A − ]

Figure 16.3 pg 762

 Buffers can also be made by mixing a weak base, (B:), with a soluble salt of its conjugate acid, H:B + Cl − H 2 O (l) + NH 3 (aq)  NH 4 + (aq) + OH − (aq)

The Henderson–Hasselbalch equation is written for a chemical reaction with a weak acid reactant and its conjugate base as a product. The chemical equation of a basic buffer is written with a weak base as a reactant and its conjugate acid as a product. B: + H 2 O  H:B + + OH − To apply the Henderson–Hasselbalch equation, the chemical equation of the basic buffer must be looked at like an acid reaction. H:B + + H 2 O  B: + H 3 O + This does not affect the concentrations, just the way we are looking at the reaction.

 Problem Set 2  Review Questions 1-9  Quiz tomorrow on  Thanksgiving Homework: ◦ Bare minimum  Notes on  Problem set 3  notes ◦ Keep in mind  Review questions due 12/1  Ch 5 packet due 12/7 (14 pages long )