pH and Buffers

C483 Spring 2013

Questions1. 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?

H+

H+

H+

H+

HClHCl

HClHCl

HCl

Cl-

Cl-

Cl-

Cl-

Cl-

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H3O+

H3O+

H3O+

H3O+

H3O+

Weak AcidsStrong 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

N

HN

NH

HN

N

HN

N

HN

N

HN

N

HN

NH

HN

NH

HN

NH

HN

NH

HN H+

H+N

HN

NH

HN

NH

HN

NH

HN

NH

HN

NH

HN

NH

HN

NH

HN

NH

HN

NH

HN

H+

H+

H+

HClHCl

HClHCl

HCl

Cl-

Cl-

Cl-

Cl-

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. F2. D3. C4. A5. E6. D