1. pH and Buffers
C483 Spring 2013

2. Questions
T/F When acid is added to pure water, Kw, the ion-product constant of water, changes.
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+.

3. 4. Two weak acids, A and B, have pKa values of 4 and 6, respectively
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.

4. Autoionization of Water

5. Kw, The Ion Product of Water

6. Reciprocal Relationship

7. pH of Neutral Water

8. 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

9. 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?

10. Weak Acids
Strong
Weak

11. Weak Acid Dissociation Constants
Weak acids have low [pdts], therefore low Ka
Low Ka = high pKa
Weaker acids have __________ Ka values and __________ pKas

13. Henderson-Hasselbalch
Proton Acceptor
Proton Donor

14. Qualitative Understanding
Relationship of
Solution pH
Strength of acid
Ratio of CB to CA
Solve quantitatively, but understand qualititively

15. Understand Figures

16. 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)

18. Buffers

20. Buffer Capacity Depends on pKa of CA/CB mix
Depends on concentration of CA/CB

21. 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

23. Blood Buffer

24. Answers F D C A E