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pH of Salt Solutions, Study notes of Molecular Structure

Basic Salts (pH > 7) are conjugate bases of weak acids ... Calculating the pH of a Solution of a Basic Salt.

Typology: Study notes

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11/11/2014
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Chapter Outline
1
15.1 Acids and Bases: The BrØnstedLowry Model
15.2 Acid Strength and Molecular Structure
15.3 pH and the Autoionization of Water
15.4 Calculations Involving pH, Ka, and Kb
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 AcidBase 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) + H2O(l) H3O+(aq) + Cl-(aq)
base conj. acid
Infinitely
strong Infinitely
weak
acid conj. base
pf3
pf4
pf5

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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

  • 14 2.9 x 10-^8

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

  • 14 1.8 x 10-^5

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]