Salts will affect the pH of a solution. Some salts will yield neutral solutions, others will yield acidic solutions, and still other salts will yield basic solutions. Below are several rules you will need to know.

Salts that Yield Neutral Solutions

A salt that contains the anion of a strong acid and the cation of a strong base will yield a neutral solution. For example, NaNO₃, will dissociated into the Na⁺ and the NO₃^– ion. The sodium ion is the cation of a strong base, NaOH, and the nitrate ion is the cation of a strong acid, HNO₃. When NaNO₃ is dissolved, the solution will be neutral because neither Na⁺ or NO₃^– will react with H₂O to any great extent.

Salts that Yield Acidic Solutions

A salt that contains the anion of a strong acid and the cation of a weak base will yield an acidic solution. One example is NH₄Cl. The NH₄⁺ ion is the cation of a weak base, NH₃, and Cl^– is the cation of a strong acid, HCl. The Cl^– will not react with water to any great extent, but the NH₄⁺ will react with water to produce H₃O⁺ ion. The solution will be acidic.

Salts that Yield Basic Solutions

A salt that contains the anion of a weak acid and the cation of a strong base will yield a basic solution. One example, NaCH₃COO, will dissociate into sodium ions and acetate ions. The sodium ion is the cation of a strong base and does not react with water, but acetate ion, CH₃COO^–, is the anion of a weak acid and reacts with water to produce hydroxide ion, OH^– ion.

Salts of Weak Acidic Cations and Weakly Basic Anions

If a salt contains the anion of a weak acid and the cation of a weak base, the pH of the solution will depend on the relative acid strength of base strength of the ions. For example, NH₄CN dissociates into NH₄⁺ ion which is the cation of a weak base, and CN^– which is the anion of a weak acid. Both will react with water:

In this case we look at K_a and K_b. The value of K_b is larger, therefore, the solution is basic. For a solution the larger K, either K_a or K_a, will determine if the solution is acidic or basic.

Exercises

Exercise 1. For the following salts, if they are dissolved in water, indicate if the solution is acidic, basic, or neutral.

Check Solution to Exercise 1

Exercise 2. What is the pH of a solution that is 2.5 M NaCN? K_{a (HCN)} = 4.9 x 10^-10

Check Solution to Exercise 2

Exercise 3. Calculate the pH of a solution that is 0.5 M Ca(ClO₄)₂.

Check Solution to Exercise 3

Exercise 4. What is the pH of an aqueous solution that contains 0.35 M NH₄NO₃?

Check Solution to Exercise 4

Exercise 5. Calculate the pH of a solution that is 0.15 M NaHSO₄.

Check Solution to Exercise 5

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