Transcript Lecture

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 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-] = 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 = [A-]/[HA]total
a = 1.2 x 10-4 /2.0 x 10-4 = 0.60
Acid – Base Equilibria
• Weak Acid Problem –
cont.:
1.00E+00
1.00E-01
Assupmption #1
Works
1.00E-02
[HA]o
– 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-03
1.00E-04
1.00E-05
Fails
1.00E-06
1.00E-07
1E-10
1E-08
1E-06
0.0001
Ka
0.01
1
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, a = 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