pH and Buffers
C483 Spring 2013
Questions
1.
T/F When acid is added to pure water, Kw, the ion-product constant of water,
changes.
2. A solution containing 10-8 M HCl and 10-8 M acetic acid contains H+ which is
supplied mostly by
A) the strong acid.
B) the weak acid.
C) both the strong and the weak acids.
D) water.
3. Compare solution A with pH = 4 to solution B with pH = 6.
A) The concentration of hydronium ion in solution A is twice that in
solution B.
B) Solution A has greater buffering capacity than solution B.
C) The concentration of hydronium ion in solution A is 100 times that
in solution B.
D) The hydroxide concentrations are equal in the two solutions since
pH only measures the concentration of H+.
4. Two weak acids, A and B, have pKa values of 4 and 6, respectively. Which statement
is true?
A) Acid A dissociates to a greater extent in water than acid B.
B) For solutions of equal concentration, acid B will have a lower pH.
C) B is the conjugate base of A.
D) Acid A is more likely to be a polyprotic acid than acid B.
E) The equivalence point of acid A is higher than that of acid B.
5. The ratio of the concentration of a ________ over ________ describes the
proportions of forms of a weak acid necessary to satisfy the Henderson-Hasselbalch
equation.
A) conjugate acid; conjugate base
B) conjugate base; conjugate acid
C) proton donor; proton acceptor
D) proton acceptor; proton donor
E) B and D
6. At the midpoint of a titration curve
A) the concentration of a conjugate base is equal to the concentration of a
conjugate acid.
B) the pH equals the pKa.
C) the ability of the solution to buffer is best.
D) All of the above.
E) A and B only.
Autoionization of Water
Kw, The Ion Product of Water
Reciprocal Relationship
pH of Neutral Water
pH of Solutions
• If acid is added to water, the
concentration of hydronium
increases and pH decreases
• If base is added to water, the
concentration of hydronium
decreases (ion product of
water) and the pH increases
• Addition of MORE acid vs.
addition of a STRONGER acid
Strong Acid
• Complete
dissociation
• What is the pH of a
0.01 M HCl
solution?
• What is your
assumption?
• You add a drop of
HCl to make a 1 x
10-8 M solution.
What is the pH?
H2O
Cl-
H2O
H2O
+
H2O
H3O+
HCl
H
H2O
HCl
H3O+
HCl
H2O
H2O
+
HCl
H3O+ H2O
ClCl-
H
HCl
H2O
H3O+
ClH2O
H2O
H
H2O
H2O
+
+
H2O
H+
H3O
H2O
H2O
Cl-
Weak Acids
Strong
Weak
Weak Acid Dissociation Constants
• Weak acids have
low [pdts],
therefore low Ka
• Low Ka = high pKa
• Weaker acids
have __________
Ka values and
__________ pKas
Henderson-Hasselbalch
Proton Acceptor
Proton Donor
Qualitative Understanding
• Relationship of
– Solution pH
– Strength of acid
– Ratio of CB to CA
• Solve quantitatively,
but understand
qualititively
Understand Figures
Problems
• Ionization state of a
moiety at a given pH
– Examples:
• Ability to read data
charts (following
page)
Less important problems
for biochemistry:
Determine pH of a weak
acid/base solution;
titration curve pH (Ignore
Calculation 2.1)
Buffers
Cl-
H
N
H
N
H
N
H
N
NH
NH
H+
NH
+
H
N
H
NH
H
N
NH
H
N
H
HCl
N
HCl
H
N
+
H
N
HCl
NH
H
N
NH
N
H
N
H
N
N
H
N
HCl
NH
HCl
Cl
H
N
H
N
NH
Cl-
-
H
N
NH
NH
H
N
N
NH
H+
H
N
Cl
N
-
N
H
N
NH
H+
H
N
NH
Cl-
Buffer Capacity
• Depends on pKa
of CA/CB mix
• Depends on
concentration of
CA/CB
Non-Important Problem
• Page 50, Sample Calculation 2.2
– Let’s Review it for concepts
• No one makes a buffer this way!
• More realistic: Describe the best way to make
a phophate buffer of pH 6.8 and 7.5
Blood Buffer
Answers
1.
2.
3.
4.
5.
6.
F
D
C
A
E
D