The Common Ion & Buffering
Common Ion Effect
• the ionization of an acid (or base) is limited
by the presence of a significant amount of
its conjugate base (or acid)
HA + H O <--->
2
• Why?
A
+ H 3O +
Common Ion Problem
1. What is the pH of 0.10 M solution of
HF? (Ka = 7.2 x10-4)
2. What is the pH of a solution that is 0.10
M HF and also contains 0.15 M F-?
BUFFERS maintain pH!
• A buffer is a solution with a __________
pH, adding acid or base to a buffer doesn’t
significantly affect the pH
• Requirements:
1) Need an acid to absorb added _____ and a
base to absorb added ____
2) The acid and base ________________ with
each other. (so conjugate pairs are used)
Suitable Buffers
• An ideal buffer is prepared from ______
amounts of a weak acid and its conjugate
_______
• The pH of a buffer ______ depends on
the Ka and can be raised or lowered by
altering the ratio of acid/base present
• The action of a buffer is special case
of the common ion effect, and allows
for a shortcut!!!!!
Equilibrium Expression
• Weak Acid:
Ka =
• Solve the expression for [H3O+]
Henderson-Hasselbalch
Equation
Weak acid and conj. base
Common Ion Problem Again
Use the H-H Equation to solve this
problem again.
1. What is the pH of a solution that is 0.10
M HF and also contains 0.15 M F-? (Ka
= 7.2 x10-4)
Henderson-Hasselbalch
Equation
• Recall that _______ buffers have equal
amounts of acid and conj. base.
• If the Ka of acetic acid is 1.8E-5, what is
the pH if a buffer is prepared having
equal concentrations of acetic acid and
acetate?
Preparing Buffer Solution
• The desired pH must be close to the
pKa of the acid
• Increase the pH by adding more
_______ and decrease the pH by
adding more _______
H-H Equation
Problem 1
• Which acid/conjugate base pair would be most
suitable to buffer a pH of 7.5?
Weak Acid
Ka
• Acetic acid
1.8 X 10-5
• Dihydrogen phosphate 6.2 x10-8
• Hydrogen phosphate 3.6 x10-13
H-H Equation
Problem 1 cont.
• What is the ratio of conjugate base to
weak acid needed to have a pH of 7.5?
H-H Equation
Problem 2
• A buffer solution contains 0.20 M acetic acid
and 0.50 M acetate. What is the pH?
Use the H-H Equation
• pH of Buffers
• pH of solutions with Common Ions
(saves you the ICE table)
• Acid/Base Titrations
Practice Problem
Problem 2 continued.
• Suppose you have 500 mL of the acetic
acid/acetate buffer system. What is the
pH after you add 25.0 mL of 1.0 M HCl
to the buffer?