pH and Buffers

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. 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 Weak Strong

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 • D