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. D) All of the above. C) the ability of the solution to buffer is best. E) A and B only.

Autoionization of Water

K

w

, 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? H 2 O H 2 O H 2 O H 2 O H + H 2 O H 2 O H 2 O H 2 O H 2 O H + H 2 O HCl HCl HCl HCl HCl Cl H 3 O + H 3 O + H + H 2 O H 3 O Cl + H 2 O Cl H 2 O Cl H 3 O + H 2 O H 2 O H 3 O + H + H 2 O Cl -

Strong

Weak Acids

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

H N H N H N NH NH N H N NH H + H N N H N NH H N N H N NH H N N H + H N N HCl HCl HCl HCl HCl Cl H N H N H N NH H + H N NH NH NH Cl H N H N H N NH NH H + H N NH Cl N Cl H N NH H N NH H + 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

1. F 2. D 3. C 4. A 5. E 6. D

Answers