V. Advanced Weak Acid and Weak Base Equilibria
V-1. Introduction to Acidic
and Basic Salts
V-2. Sample Problem V-3. List of Problems V-4. Practice Problems

Introduction

A salt is an ionic compound that contains at least one spectator ion. The salt therefore dissociates completely in water. Memorize the spectator ions if you haven't already. Knowing them allows you to identify all salts, strong acids, and strong bases. Weak acids are compounds that only partially dissociate to produce H+, and weak bases partially react with water to produce OH-.

Some salts will produce an acidic or basic solution when added to water. The key to understanding these type of problems is to identify the ions and what they could do. Spectator ions, as their name implies, do not take part in any equilibrium. Some cations will act as weak acids by partially dissociating to produce H+. Some anions will act as weak bases by partially reacting with water to produce OH-. The following table provides a reminder of the spectator ions, and lists some acidic and basic ions of common salts.

SpectatorBasicAcidic
AnionsCl-   Br-   I-
NO3-   ClO4-   SO42-
F-   CO32-   PO43-
CH3COO- and other organics
 
CationsLi+   Na+   K+   Rb+
(Ca2+)   Sr2+   Ba2+
 NH4+   Al3+   Mg2+
transition metals

(Ca2+ is a borderline case. Many calcium salts are soluble at moderate concentrations, but at higher Ca2+ concentrations or high pH a Ca(OH)2 precipitate can form.)

To determine the effect of the salt when it dissolves in water:

  1. Predict the effect of the cation on the pH of water.
  2. Predict the effect of the anion on the pH of water.
  3. Determine the overall effect.
The tricky problems are the salts that contain both an acidic cation and a basic anion. In these cases, the solution will be acidic if Ka (of the cation) > Kb (of the anion) and basic if Kb > Ka.


Examples

Adding NH4Cl to water.
When you identify a species as a salt, remember that it will completely dissociate in water:
NH4Cl(s) ---> NH4+(aq) + Cl-(aq)

NH4+ is acidic and Cl- is a spectator ion, so the solution will be acidic:
NH4+(aq) + H2O <--> NH3 (aq) + H3O+(aq)

Adding sodium acetate to water.
Na+ is a spectator ion and CH3COO- is basic, so the solution will be basic:
CH3COO-(aq) + H2O <--> CH3COOH(aq) + OH-(aq)

Adding NH4F to water.
NH4+ is acidic and F- is basic. The Ka of NH4+ is 5.6x10-10 and Kb of F- is 1.4x10-11. Ka > Kb so the solution will be acidic.

Adding NaHCO3 (sodium bicarbonate) to water.
HCO3-(aq) + H2O <--> CO32-(aq) + H3O+(aq) Ka = 4.7x10-11
HCO3-(aq) + H2O <--> H2CO3 (aq) + OH-(aq) Kb = 2.3x10-8

Kb > Ka so this solution will be basic.

Quantitative calculation of the equilibria of acidic and basic salts is similar to weak acids and weak bases. Before trying the salt problems, be sure you completely understand weak acid and weak base equilibria.

For many of these problems you won't know the Ka or Kb, but you will know the Kb or Ka, respectively, of the conjugate base or conjugate acid. To find the equilibrium constant that you need, use the relationship Ka * Kb = 1.00x10-14.


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