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