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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+.
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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
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Kw, The Ion Product of Water
Reciprocal Relationship
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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
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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
H2O
Cl-
H2O
H2O
H+
H3O+
HCl
H2O
HCl
H3O+
HCl
HCl
H2O
H2O
H+
HCl
H2O
ClH2O
H 2O
H3O+ H2O
ClCl-
H2O
H2O
H+
H3O+
H2O
H3O+
H2O
H+
H2O
Cl-
Weak Acids
Strong
Weak
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Weak Acid Dissociation Constants
• Weak acids have low [pdts], therefore low Ka
• Low Ka = high pKa
• Weaker acids have __________ Ka values and __________ pKas
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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
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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)
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Buffers
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Cl-
H
N
H
N
NH
NH
H+
H
N
H
N
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
NH
HCl
Cl
Cl-
H
N
H
N
H
N
NH
NH
H
N
N
NH
+
H
Cl-
H
N
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
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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 11
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Blood Buffer
Answers
1.
2.
3.
4.
5.
6.
F
D
C
A
E
D
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