1. Lecture

2. Acid – Base Equilibria Weak Acid Problem Example: –What is pH and the concentration of major species in a 2.0 x 10 -4 M HCO 2 H (formic acid, K a = 1.80 x 10 -4 ) solution ? –Approaches: Systematic Method: –too complicated for a “complete” solution –with assumptions in charge balance equation ([H + ] = [A - ] + [OH - ] goes to either [H + ] = [A - ] or [H + ] = [OH - ]), problem can be solved easily ICE Method –can be used in “most cases” (or specifically when the charged balance equation can be simplified to [H + ] = [A - ])

3. Acid – Base Equilibria Weak Acid Problem – Example Results x = 1.2 x 10 -4 M = [H + ] = [A - ] (using either systematic method with assumption or ICE method) Note: we know assumption [H + ] = [A - ] is valid since [OH - ] = K w /[H + ] = 8 x 10 -11 M << [A - ] [HA] = [HA] o – [A - ] = 2.0 x 10 M - 1.2 x 10 -4 M [HA] = 8 x 10 -5 M Fraction of Dissociation =  = [A - ]/[HA] total  = 1.2 x 10 -4 /2.0 x 10 -4 = 0.60

4. Acid – Base Equilibria Weak Acid Problem – cont.: –When is Assumption #1 ([H + ] = [A - ]) valid (in general)? –When both [HA] o and K a are high or so long as [H + ] > 10 -6 M –More precisely, when [HA] o > 10 -6 M and K a [HA] o > 10 -12 –See chart (shows region where error < 1%) Assupmption #1 Works Fails

5. 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 NH 3 solution (K a (NH 4 + ) = 5.7 x 10 -10, so K b = K w /K a = 1.75 x 10 -5 ) –Reaction NH 3 + H 2 O  NH 4 + + OH - –You can do the rest –For bases,  = fraction of dissociation = [BH + ]/[B] o

6. Acid – Base Equilibria Buffer Solutions: –A buffer solution is designed so that a small addition of acid or base will only slightly change the pH –Most buffer solutions have a weak acid and its conjugate base both present –Example: Determine pH of a mix of 0.010 M HCO 2 H and 0.025 M Na + HCO 2 - solution (ignoring activity) –Go to board to show if ICE approach is needed