Chapter 15
Applications of
Aqueous Equilibria
The Common-Ion Effect
Common-Ion Effect: The shift in the position of an equilibrium on addition
of a substance that provides an ion in common with one of the ions
already involved in the equilibrium
This applies to weak acids, weak bases and solubility of salts when a
common ion is added to the equilibrium reaction. You can change the
pH, by adding salt.
HF (aq) + H2O(aq)
H3O+ (aq) + F- (aq)
Ka = 7.2 x 10-4
What happened to the equilibirium as NaF salt was added to the solution?
NaF (s) → Na+ (aq) + F– (aq)
When F- is added (from NaF), then [H+] must decrease (Le
Chatelier’s principle). The pH increases
15.3 Buffer Solutions
Buffer Solution: A solution which contains a weak acid
and its conjugate base and resists drastic changes in pH.
Weak acid
+
Conjugate base
For
Example:
CH3CO2H + CH3CO21HF + F1NH41+ + NH3
H2PO41- + HPO42-
Buffer Solutions: contain a common ion and are important in
biochemical and physiological processes
Organisms (and humans) have built-in buffers to protect them
against changes in pH.
Human blood is maintained by a combination of CO3-2, PO4-3
and protein buffers.
Blood: (pH 7.4)
Death = 7.0 <pH > 7.8 = Death
Example

Calculate the pH of a solution that is prepared by dissolved 0.10 mol of
acetic acid and 0.10 mol sodium acetate in enough water to make 1.00 L
of solution. Ka = 1.8 x 10-5
Example

Calculate the pH of 0.15M HF and 0.25NaF mixture. Is this a
buffer solution?
Example

Calculate the pH of 100.0 mL DI water

Calculate the new pH after adding 1.0 mL of 0.10M HCl to the
above water solution.
Buffer Solutions
HA(aq) + H2O(l)
H3O1+(aq) + A1-(aq)
Weak acid

Conjugate base
(M+A-)
Add a small amount of base (-OH) to a buffer solution
◦ Acid component of solution neutralizes the added base
Addition of OH1- to a buffer:
HA(aq) + OH1-(aq)
100%
H2O(l) + A1-(aq)
Buffer Solutions
HA(aq) + H2O(l)
H3O1+(aq) + A1-(aq)
Conjugate base
(M+A-)
Weak acid

Add a small amount of acid (H3O+) to a
buffer solution
◦ Base component of solution
neutralizes the added acid
H3O+ added
Addition of H3O1+ to a buffer:
A1-(aq)
+
H3O1+(aq)
100%
H2O(l) + HA(aq)
The H3O+ added
changed A- to HA,
but [HA] and [A-]
are large compared
to the [H3O+] added
Buffer Solutions

The addition of –OH or H3O+ to a buffer solution will change
the pH of the solution, but not as drastically as the addition of
–OH or H O+ to a non-buffered solution
3
Example

pH of human blood (pH = 7.4) controlled by conjugated acid-base pairs
(H2CO3/HCO3-). Write an equation for this buffer mixture then
neutralization equation for the following effects
◦ With addition of HCl
◦ With addition of NaOH
Example

50.0 mL of 0.100 M HCl was added to a .100L buffer consisting
of 0.025 moles of sodium acetate and 0.030 moles of acetic acid.
What is the pH of the buffer before and after the addition of the
acid? Ka of acetic acid is 1.7 x 10-5. Assume the volume is
constant
Example

Calculate the pH of 0.100L of a buffer solution that is 0.25M in HF
and 0.50 M in NaF.
◦ What is the change in pH on addition of 0.010 moles KOH
◦ Calculate the pH after addition of 0.080 moles HBr
• Assume the volume remains constant
• Ka = 3.5 x 10-4
Example
Calculate the pH of the buffer that results from mixing 60.0mL of
0.250M HCHO2 and 15.0 mL of 0.500M NaCHO2
Ka = 1.7 x 10-4
◦ Calculate the pH after addition of 10.0 mL of 0.150 MHBr.
Assume volume is additive

Buffer Capacity





A measure of amount of acid or base that the solution can
absorb without a significant change in pH.
Depends on how many moles of weak acid and conjugated base
are present.
For an equal volume of solution: the more concentrated the
solution, the greater buffer capacity
For an equal concentration: the greater the volume, the
greater the buffer capacity
The capacity of a buffer depends on the total (formal)
concentration of the buffer species ( [acid] + [conjugate base] ),
and for a given total concentration, buffer capacity will be
greatest when [acid] = [conjugate base] that is, when pH = pKa
for that buffer's acidic form.
Example

The following pictures represent solutions that contain a weak
acid HA and/ or its sodium salt NaA. (Na+ ions and solvent water
molecules have been omitted for clarity

Which of the solutions are buffer solution?
Which solution has the greatest buffer capacity?

Example

What is the maximum amount of acid that can be added to a
buffer made by the mixing of 0.35 moles of sodium hydrogen
carbonate with 0.50 moles of sodium carbonate? How much
base can be added before the pH will begin to show a
significant change?
15.4 The Henderson-Hasselbalch
Equation
Weak acid
Conjugate base
Acid(aq) + H2O(l)
Ka =
[H3O1+][Base]
[H3O1+] = Ka
[Acid]
pH = pKa + log
H3O1+(aq) + Base(aq)
[Base]
[Acid]
[Acid]
[Base]
Examples

Calculate the pH of a buffer solution that is 0.50 M in benzoic
acid (HC7H5O2) and 0.150 M in sodium benzoate (NaC7H5O2).
Ka = 6.5 x 10-5
Example

How would you prepare a NaHCO3-Na2CO3 buffer solution that
has pH = 10.40 Ka2 = 4.7 x 10-11
Example

You prepare a buffer solution of .323 M NH3 and (NH4)2SO4.
What molarity of (NH4)2SO4 is necessary to have a pH of 8.6?
(pKb NH3= 4.74)