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Lecture
Acid – Base Equilibria
• Weak Acid Problem Example:– What is pH and the concentration of major
species in a 2.0 x 10-4 M HCO2H (formic acid, Ka = 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-])
Acid – Base Equilibria
• Weak Acid Problem – Example Resultsx = 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-] = Kw/[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
Acid – Base Equilibria
• Weak Acid Problem – cont.:– When is Assumption #1
([H+] = [A-]) valid (in general)?
– When both [HA]o and Ka are high or so long as [H+] > 10-6 M
– More precisely, when [HA]o > 10-6 M and Ka[HA]o > 10-12
– See chart (shows region where error < 1%)
Where ICE Method Works
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
1E-10 1E-08 1E-06 0.0001 0.01 1
Ka
[HA
]o
Assupmption #1 Works
Fails
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-
– You can do the rest– For bases, = fraction of dissociation =
[BH+]/[B]o
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 HCO2H and 0.025 M Na+HCO2
- solution (ignoring activity)
– Go to board to show if ICE approach is needed