1. Chem. 31 – 11/29 Lecture

2. Announcements GC Lab Due Today
Co/Cr Lab – grading delay – due to problem not with grader (thanks for your patience)
Today’s Lecture
Chapter 9 – Acid/Base Equilibria
The weak base problem
Buffers

3. Acid – Base Equilibria Weak Base Problem:
As with weak acid problem, ICE approach can generally be used (except when [OH-] from base is not much more than [OH-] from water)
Note: when using ICE method, must have correct reaction
Example: Determine pH of M NH3 solution (Ka(NH4+) = 5.7 x 10-10, so Kb = Kw/Ka = 1.75 x 10-5)
Reaction
NH3 + H2O  NH4+ + OH-
Go over on board 3

4. Acid – Base Equilibria Weak Acid/Weak Base Questions:
A solution is prepared by dissolving 0.10 moles of NH4NO3 into water to make 1.00 L of solution. Show how to set up this problem for determining the pH using the ICE method.
A student is solving a weak base problem for a weak base initially at 1.00 x 10-4 M using the ICE method and calculates that [OH-] = 2.4 x 10-8 M. Was the ICE method appropriate?
The pH of an unknown weak acid prepared to a concentration M is measured and found to be Calculate a and Ka. 4

5. 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 < pKa < 11)
Ratio of weak acid to conjugate base in range 0.1 to 10
mM+ concentration range 5

6. Acid – Base Equilibria Buffer Solutions:
Since ICE not needed, can just use Ka equation
Ka = [H+][A-]/[HA] = [H+][A-]o/[HA]o
(always valid) (valid for traditional buffer)
But log version more common
pH = pKa + log([A-]/[HA])
Also known as Henderson-Hasselbalch Equation 6

7. Acid – Base Equilibria Buffer Solutions: Why are they needed?/useful?
The main reason is to keep the pH constant so that the ratio of species of acids and bases is constants
Some examples:
in water hardness titration, we want [Y4-]/[Y]total constant so sample pH won’t affect results
spectroscopy: Beer’s law only applies to single species (e.g. separate laws for HIn and In-)
chromatography: at pH = 2, benzoic acid is a molecule and retained (reversed-phase HPLC) 7

8. Acid – Base Equilibria Buffer Solutions: Ways to make buffer solution:
Mix weak acid and conjugate base (done in making reference solution for soda ash lab)
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 8

9. Acid – Base Equilibria Example Problems:
How many moles of hydroxyl ammonium chloride (HONH3+Cl-) needs to be added to 500 mL of M HONH2 to obtain a buffer solution with a pH of 6.20? The pKa for HONH3+ is 5.96.
What is the pH of a solution made from mixing 400 mL of M CH3CO2H (pKa = 4.75) with 100 mL of M NaOH? (assume additive volumes) 9

10. Acid – Base Equilibria Example Qualitative Question:
Which of the following mixtures will result in a traditional buffer:
0.010 M CH3CO2H M KOH
0.014 M NH4Cl M KOH
0.020 M NH2CH2CH2OH M NaOH
0.010 M Na2HPO x 10-4 M Na3PO4
NH2CH2CH2OH is a weak base 10