Acids and Bases: Salts
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Introduction
Molecular Structure
Ionization Constants
Salts
Buffers
Lewis Theory

GOAL(s):

  • Recognize salts as products as acid-base reactions
  • Determine if an aqueous salt solution will be acidic, basic, or neutral
  • Understand how pH can affect solubility

The ingredient list for almost any household product is sure to inlude at least one compound that chemists classify as a salt. While you may think of salt as the white granules used to flavor foods (known by chemists as sodium chloride, NaCl), chemists classify many compounds as salts. To chemists, a salt is any ionic compound that could have been formed by an acid-base reaction. Sodium chloride fits this definition since it could be formed by the reaction of hydrochloric acid and sodium hydroxide:

HCl + NaOH NaCl + H2O

When any acid and base are combined, an exchange reaction occurs, producing a salt and water. A generic equation for this process is shown below:

HA + BOH BA + H2O

The salt's negative ion (A-) is the conjugate base of the acid HA while the salt's positive ion (B+) is the conjugate acid of the base BOH. Depending on the strength of the acid and base, the resulting solution may be acidic, basic, or neutral.

If the strengths of the original acid (HA) and base (BOH) are known, the strengths of their conjugate acid and base can be determined since the ionization constants of conjugate acid-base pairs are related:

Ka x Kb = Kw = 1.0 x 10-14

Complete the following statements:

1

 If HA is a strong acid, A- will be a base
   and will cause the pH of the solution to

2

 If HA is a weak acid, A- will be a base
   and will cause the pH of the solution to

Good! This relationship is also seen with bases and their conjugate acids. If BOH is a very strong base, B+ will be a very weak acid and will not affect the pH of a solution. If BOH is a weak base, B+ will be a weak acid and will cause the pH of a solution to decrease. These relationships are summarized in the table below:

Example Conjugate base Strength of
conjugate base		 Affect on pH
Strong acid HNO3  NO3- Very weak None
Weak acid HCO2H  HCO2- Weak Increase
Example Conjugate acid Strength of
conjugate acid		 Affect on pH
Strong base  KOH  K+ Very weak  None 
Weak Base NH3  NH4+ Weak Decrease

A strong acid is one that completely ionizes in water. Its ionization constant is too large to measure. If this is the case, the conjugate base will have an ionization constant that is incredibly small (too small to measure!) and can be classified as very weak.

Suppose a weak acid has an ionization constant of 1.0 x 10-4. The ionization constant of its conjugate base would be 1.0 x 10-10. How would you classify the strength of this base? Now suppose a weak acid has an ionization constant of 1.0 x 10-11. The ionization constant of its conjugate base would be 1.0 x 10-3. Would the strength of this base be in the same classification as the first example?

Remember that a base (unless it is very weak) will cause the pH of a solution to increase.

A very weak base will not affect the pH of the solution.

Salts