Acids and Bases. Arrhenius Acids and Bases

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Slide 20

Recall for problem solving:

****[H+] is the same as [H3O+]

1) pH = -log [H+]

2) [H+] = 10-pH or antilog (-pH) = [H+]

3) pOH = -log [OH-]

4) [OH-] = 10-pOH or

antilog (-pOH-) = [OH-]

5) [H+] [OH-] = 1 x 10-14

6) pH + pOH = 14

Slide 21

Problem Solving:

1) A blood sample (considered a dilute aqueous solution) contains 7.2 x 10-8 mol H+ per liter.

a. What is the pH of the blood sample?

b. What is the [OH-] of the blood sample?

c. Is the blood sample acidic, basic, or neutral?

2) The pH of an aqueous solution is 8.7

a. What is the [H+]?

b. What is the pOH?

Slide 22

Strong Acids and Bases

Strong acids and bases

undergo essentially complete

dissociation (ionization).

Equilibrium expressions not

needed for problem solving.

Example: What is the pH and [H+] for a

0.05 M HCl solution?

Slide 23

Problem Solving for Weak Acids and Bases

1. They only dissociate or ionize partially in solution.

2. This means we have an equilibrium situation, and can solve problems using the techniques we learned earlier for dealing with equilibrium problems. (ICE tables…)

3. Equilibrium Constants (Kc)

Ka for weak acids

Kb for weak bases

Slide 24

Weak Acid Equilibrium Rxns

A. Acid Ionization Constant

1. Definition

Give the acid ionization constant expression for

the acid: HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

or HA(aq) ⇌ H+(aq) + A-(aq)

Ka = ?

Slide 25

2. What does the Ka tell you?

Larger Ka  stronger acid

(more of the acid ionizes in water to form H+)

See Table 15.5, for examples

Which is the strongest acid?

Acetic acid Ka = 1.8 x 10-5

Boric Acid Ka = 7.3 x 10-10

Slide 26

Slide 27

B. Ka From Equilibrium Concentrations

A 0.10 M solution of phenol (weak acid) has a pH of 5.43. Calculate the Ka for phenol.

Slide 28

C. pH Calculations from Ka of Weak Acid

The weak monoprotic acid, parahydroxybenzoic acid, has a Ka of 2.6 x 10-5. Answer the following questions for a 0.200 M solution of parahydroxybenzoic acid.