1. Ch. 19 - Strength of Acids & Bases Ch. 19 – Strengths of Acids & Bases
19.3 pp. 602 – 611
19.3 pp

2. HA(aq) + H2O(l) → H3O+(aq) + A-(aq)
A. Strong & Weak Acids
Strong acids are completely ionized in aqueous solution
HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

3. A. Strong & Weak Acids
Weak acids ionize only slightly in aqueous solution
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq)
~99% ~1%
Initial Amounts
Equilibrium Amounts

4. B. Acid Dissociation Constant
You can write the equilibrium-constant expression from balanced chemical equation
CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq)
The acid dissociation constant, Ka, is ratio of concentrations of dissociated and undissociated form

5. B. Acid Dissociation Constant
Reflects the fraction of the acid in the ionized form
If Ka is small, then degree of dissociation is small
Weak acids have small Ka values
The stronger an acid is, the larger its Ka value
Often written in generic form:
HA  H+ + A–

6. NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
C. Strong & Weak Bases
Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution
Weak bases react with water to form the hydroxide ion and the conjugate acid of the base
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

7. D. Base Dissociation Constant
You can write the equilibrium-constant expression from balanced chemical equation
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
The base dissociation constant, Kb, is ratio of concentrations of dissociated and undissociated form

8. D. Base Dissociation Constant
Size of Kb indicates ability of weak base to compete with very strong base OH– for hydrogen ions
Weak bases have small Kb values
The stronger a base is, the larger its Kb value
Often written in generic form:
B + H2O  BH+ + OH–

9. E. Calculating Dissociation Constants
A 0.100M solution of acetic acid is only partially ionized. From the pH, [H+] is determined to be 1.34 x 10-3M. What is Ka of acetic acid?
Brackets indicate concentrations at equilibrium – you need to find equilibrium values!

10. E. Calculating Dissociation Constants
Concentration
[CH3COOH]
[H+]
[CH3COO-]
Initial
0.100
Change
-1.34 x 10-3
+1.34 x 10-3
Equilibrium
0.0987
1.34 x 10-3

11. F. Percent Dissociation
Often useful to specify amount of weak acid or base that has dissociated in achieving equilibrium
Ex) 1.00M solution of HF, [H+] = 2.7x10-2 M

12. G. Weak Acids and Bases WS
Find the pH and pOH of a 0.325M acetic acid solution. Ka = 1.8 x 10-5
HA  H A–
Concentration
[CH3COOH]
[H+]
[CH3COO-]
Initial
0.325
Change -x +x
Equilibrium x x

13. G. Weak Acids and Bases WS
Find the pH and pOH of a 0.325M acetic acid solution. Ka = 1.8 x 10-5