Chem. 31 – 11/29 Lecture

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

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 0.010 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

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 0.0100 M is measured and found to be 3.77. Calculate a and Ka. 4

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

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

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

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

Acid – Base Equilibria Example Problems: How many moles of hydroxyl ammonium chloride (HONH3+Cl-) needs to be added to 500 mL of 0.020 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 0.018 M CH3CO2H (pKa = 4.75) with 100 mL of 0.024 M NaOH? (assume additive volumes) 9

Acid – Base Equilibria Example Qualitative Question: Which of the following mixtures will result in a traditional buffer: 0.010 M CH3CO2H+ 0.014 M KOH 0.014 M NH4Cl + 0.010 M KOH 0.020 M NH2CH2CH2OH + 0.010 M NaOH 0.010 M Na2HPO4 + 1.0 x 10-4 M Na3PO4 NH2CH2CH2OH is a weak base 10