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Week 10 CHEM 1310 - Sections L and M 1

Ch. 7 - Acids and Bases cont.

Last Week Arrhenius vs. Bronsted-Lowry Acids and Bases Conjugate Acid-Base Pairs Water: Acid or Base pH and Related Simple Calculations

Today Acid Strength More pH Calculations Bases (more detail) Polyprotic Acids (more detail)

Week 10 CHEM 1310 - Sections L and M 2

Determining Acid StrengthVia Ka Value

The larger the Ka value the stronger the acid

Week 10 CHEM 1310 - Sections L and M 3

Determining Acid StrengthVia Position of Equilibrium

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Strong Acidsfully dissociate!

Before equilibrium At equilibrium

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Week 10 CHEM 1310 - Sections L and M 4

Determining Acid Strength

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

[HA]0 ≈ [H+]equilibriumfor

Strong Acids

Before equilibrium At equilibrium

Via Equilibrium [H+] vs. Original [HA]

Week 10 CHEM 1310 - Sections L and M 5

Determining Acid StrengthVia Position of Equilibrium

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Weak Acidsdo not fullydissociate!

Before equilibrium At equilibrium

Week 10 CHEM 1310 - Sections L and M 6

Determining Acid Strength

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

[HA]0 >> [H+]equilibriumfor

Weak Acids!

Before equilibrium At equilibrium

Via Equilibrium [H+] vs. Original [HA]

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Week 10 CHEM 1310 - Sections L and M 7

Determining Acid StrengthVia Strength of A- vs. H2O

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Strongacid

Weak Conjugate

Base

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Weakacid

Strong Conjugate

Base

Week 10 CHEM 1310 - Sections L and M 8

PRS Question

1. Weak Acid

2. Strong Acid

3. HF is not an acid

4. Cannot be determined from information given

Is HF a strong or a weak acid?

Acid Formula Ka

Hydrochloric Acid HCl 1 x 107

Hydrofluoric Acid HF 7.2 x 10-4

Week 10 CHEM 1310 - Sections L and M 9

PRS Question

1. Weak Acid

2. Strong Acid

3. HF is not an acid

4. Cannot be determined from information given

Is HF a strong or a weak acid?

The larger the Ka valuethe stronger the acid

Acid Formula Ka

Hydrochloric Acid HCl 1 x 107

Hydrofluoric Acid HF 7.2 x 10-4

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Week 10 CHEM 1310 - Sections L and M 10

pH: Strong vs. Weak Acids

pH = -log [H+]

For a strong acid: [HA] ≈ [H+] = 1.0 M

Thus, pH = -log (1.0) = 0

What is the pH of a 1.0M HCl solution?

Acid Formula Ka

Hydrochloric Acid HCl 1 x 107

Hydrofluoric Acid HF 7.2 x 10-4

Week 10 CHEM 1310 - Sections L and M 11

pH: Strong vs. Weak Acids

pH = -log [H+]What is the pH of a 1.0M HF solution?

HF H+ + F-

Ka =[H+] x [F-]

[HF]Ka =

X2

[HF]0 - X

[HF]0 >> X

Acid Formula Ka

Hydrochloric Acid HCl 1 x 107

Hydrofluoric Acid HF 7.2 x 10-4

Week 10 CHEM 1310 - Sections L and M 12

pH: Strong vs. Weak Acids

pH = -log [H+]What is the pH of a 1.0M HF solution?

HF H+ + F-

Ka =X2

1 - X

[HF]0 >> X

Let X = [H+] = [F-]

X = √Ka = √7.2 x 10-4 X = 0.0268

Ka = X2

Acid Formula Ka

Hydrochloric Acid HCl 1 x 107

Hydrofluoric Acid HF 7.2 x 10-4

pH = -log (0.0268) = 1.6

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Week 10 CHEM 1310 - Sections L and M 13

Another Sample Problem

A solution is prepared by dissolving 0.56-gof benzoic acid (C6H5COOH; Ka = 6.4x10-5)into enough water to make 1 L.Calculate the pH of the solution.

Determine the MW of benzoic acid Write the chemical equation describing the

dissolution of this acid Is this a weak acid?

Solve problem on board!

Week 10 CHEM 1310 - Sections L and M 14

PRS Question

Which of the following Ka values belongs tothe strongest acid?

1. 6.6 x 10-4

2. 4.6 x 10-4

3. 9.1 x 10-8

4. 3.0 x 10-8

5. Cannot be determined from the info given

Week 10 CHEM 1310 - Sections L and M 15

PRS Question

Which of the following Ka values belongs tothe strongest acid?

1. 6.6 x 10-4

2. 4.6 x 10-4

3. 9.1 x 10-8

4. 3.0 x 10-8

5. Cannot be determined from the info given

The stronger the acid,the higher the Ka

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Week 10 CHEM 1310 - Sections L and M 16

PRS Question

Which of the following pKa values belongs tothe strongest acid?

1. 9.25

2. 7.46

3. 3.40

4. 1.92

5. Cannot be determined from the info given

Week 10 CHEM 1310 - Sections L and M 17

PRS Question

Which of the following pKa values belongs tothe strongest acid?

1. 9.25

2. 7.46

3. 3.40

4. 1.92

5. Cannot be determined from the info given

The stronger the acid,the higher the Ka

and the lower the pKa!

Week 10 CHEM 1310 - Sections L and M 18

Percent DissociationHA(aq) + H2O(l) H3O+(aq) + A-(aq)

% Dissociation = [A-][HA]

x 100

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Week 10 CHEM 1310 - Sections L and M 19

Bases and Base Strength

B (aq) + H2O (l) ↔ BH+ (aq) + OH– (aq)B = Base

Kb is the Basicityconstant

b

2

[BH ][OH ]K

[B][H O]

[BH ][OH ]

[B]

+ -

+ -

=

=

b 10 bpK log K= -

Week 10 CHEM 1310 - Sections L and M 20

Polyprotic Acids

Ka1 > Ka2 > Ka3

Week 10 CHEM 1310 - Sections L and M 21

Polyprotic Acids

Sulfuric acid (H2SO4) is unique Ka1 = 100 STRONG ACID Ka2 = 1.2 x 10-2 WEAK ACID

H2SO4 (aq) → H+ (aq) + HSO4– (aq) Ka1 ≈ 1.0x102

HSO4– (aq) ↔ H+ (aq) + SO4

2– (aq) Ka2 ≈ 1.2x10-2

4a1

a2

K 10

Kª