01:160:159 Lecture Notes - Lecture 15: Hydrofluoric Acid, Buffer Solution, Weak Base
Chapter 15 - Applications of Aqueous Equilibria
Acid-Base Equilibria
15.1 Solutions of Acids or Bases Containing a Common Ion
A. Common Ion
1. Ion provided in solution by an aqueous acid (or base) as well as a salt
a. HF(aq) and NaF (F- in common)
HF(aq) Ë H+(aq) + F-(aq) Excess F- added by NaF
Equilibrium shifts away from added
component. Fewer H+ ions present.
pH is higher than expected.
b. NH4OH and NH4Cl (NH4+ in common)
NH3(aq) + H2O(l) Ë NH4+(aq) + OH-(aq)
Equilibrium shifts to the left. pH of the solution
decreases due to a decrease in OH- concentration
B. Equilibrium Calculations
1. Consider initial concentration of ion from salt when calculating values for
H+ and OH-
15.2 Buffered Solutions
A. Buffered Solution
1. A solution that resists a change in pH when either hydroxide ions or
protons are added.
2. Buffered solutions contain either:
a. A weak acid and its salt
b. A weak base and its salt
B. Calculations Involving Buffered Solutions Containing Weak Acids
1. "Buffered solutions are simply solutions of weak acids or bases containing
a common ion. The pH calculations on buffered solutions require exactly
the same procedures introduced in Chapter 14. This is not a new type of
problem."
2. "When a strong acid or base is added to a buffered solution, it is best to
deal with the stoichiometry of the resulting reaction first. After the
stoichiometric calculations are completed, then consider the equilibrium
calculations. This procedure can be presented as follows:
1
C. Buffering: How Does it Work?
1. OH- + HA → A- + H2O
weak conjugate
acid base
OH- ions are not allowed to accumulate but are replaced by A- ions
2. K
=
[H
+
][ A
−
]
[H
+
] = K
[HA]
a
[HA]
a [ A− ]
a. If the amounts of HA and A- originally present are very large
compared with the amount of OH- added, the change in [HA]/[A-]
will be small. Therefore the pH change will be small
3. Buffering also works for addition of protons instead of hydroxide
ions H+ + A- → HA
ConjugateWeak
Baseacid
2
