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Chapter Outline
1
- 15.1 Acids and Bases: The BrØnsted–Lowry Model
- 15.2 Acid Strength and Molecular Structure
- 15.3 pH and the Autoionization of Water
- 15.4 Calculations Involving pH, K a, and K b
- 15.5 Polyprotic Acids
- 15.6 pH of Salt Solutions
- 15.7 The Common-Ion Effect
- 15.8 pH Buffers
- 15.9 pH Indicators and Acid–Base Titrations
- 15.10 Solubility Equilibria
pH of Salt Solutions
1. Neutral Salts (pH = 7) are from strong electrolytes (100% ionization)
(a)Ionic Compounds:
NaCl(aq) Na+(aq) + Cl-(aq)
HCl(aq) + H 2 O(l) H 3 O+(aq) + Cl-(aq)
base conj. acid
Infinitely strong
Infinitely weak
acid conj. base
pH of Salt Solutions
2. Basic Salts (pH > 7) are conjugate bases of weak acids
HClO(aq) + H 2 O(l) = H 3 O+(aq) + ClO-(aq)
ClO-(aq) + H 2 O(l) = OH-(aq) + HClO(aq)
weak acid
conj. base
Ka (HClO) = 2.9 x 10-^8 Kb = Kw Ka
Kb = 1.0 x 10
conj. base pH > 7
Kb (ClO-) = 3.4 x 10-^7
3. Acidic Salts (pH < 7) are conjugate acids of weak bases
pH of Salt Solutions
NH 3 (aq) + H 2 O(l) = OH-(aq) + NH 4 +(aq)
NH 4 +(aq) + H 2 O(l) = H 3 O+(aq) + NH 3 (aq)
weak base
conj. acid
Kb (NH 3 ) = 1.8 x 10-^5 Ka =^ Kw Kb
Ka = 1.0 x 10
conj. acid pH < 7
Ka (NH 4 +) = 5.6 x 10-^10
Calculating the pH of Solutions of Weak Acids and Bases: Use the RICE Table as Before
2. Calculating the pH of a Solution of an Acidic Salt (Ex. 15.8)
What is the pH of a 0.25 M solution of NH 4 Cl?
Chapter Outline
8
- 15.1 Acids and Bases: The BrØnsted–Lowry Model
- 15.2 Acid Strength and Molecular Structure
- 15.3 pH and the Autoionization of Water
- 15.4 Calculations Involving pH, K a, and K b
- 15.5 Polyprotic Acids
- 15.6 pH of Salt Solutions
- 15.7 The Common-Ion Effect
- 15.8 pH Buffers
- 15.9 pH Indicators and Acid–Base Titrations
- 15.10 Solubility Equilibria
The Common Ion Effect
A shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.
CH 3 COONa ( s ) Na+^ ( aq ) + CH 3 COO-^ ( aq )
CH 3 COOH ( aq ) H+^ ( aq ) + CH 3 COO-^ ( aq )
common ion
The common ion effect can be used to produce a BUFFER SOLUTION = a solution of a weak acid or base and it's conjugate, e.g. CH 3 COOH and CH 3 COONa
By controlling the ratio of weak acid/base to it's conjugate, we can shift the equilibrium to whatever [H+] and therefore pH we want.
The Henderson-Hasselbach Equation
HA(aq) + H 2 O(aq) = H 3 O+(aq) + A-(aq)
Ka =
[H 3 O+][A-]
[HA]
