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.

(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.

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 NH_{3 (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 K_a of NH₄⁺ is 5.6x10^-10 and K_b of F^- is 1.4x10^-11. K_a > K_b so the solution will be acidic.

Adding NaHCO₃ (sodium bicarbonate) to water.
HCO₃^-_(aq) + H₂O CO₃^2-_(aq) + H₃O⁺_(aq) K_a = 4.7x10^-11
HCO₃^-_(aq) + H₂O H₂CO_{3 (aq)} + OH^-_(aq) K_b = 2.3x10^-8

K_b > K_a 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 K_a or K_b, but you will know the K_b or K_a, respectively, of the conjugate base or conjugate acid. To find the equilibrium constant that you need, use the relationship K_a * K_b = 1.00x10^-14.