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9.8
THE ACIDITY AND BASICITY OF SALT SOLUTIONS

Thus far we have discussed what occurs when an acid or a base dissolves in water. What happens to the pH of water when a salt is dissolved? A salt is the product of a neutralization reaction (Section 9.7A). Does this mean that a salt dissolved in water forms a neutral solution with a pH of 7?

Let's first examine what acid and base are used to form a salt M+A.

Thus, we consider NaCl to be a salt formed when the strong base NaOH and the strong acid HCl react together. On the other hand, NaHCO3 is formed from the strong base NaOH and the weak acid H2CO3, and NH4Cl is formed from the weak base NH3 and the strong acid HCl.

HEALTH NOTE

Like taking other over-the-counter medications, care must be exercised when using antacids. Ingestion of large amounts of CaCO3 can increase the incidence of kidney stones.

A salt derived from a strong base and a strong acid forms a neutral solution (pH = 7). When one ion of a salt is derived from a weak acid or base, one principle is followed: the ion derived from the stronger acid or base determines whether the solution is acidic or basic.

For example, when NaHCO3dissolves in water, it forms Na+(aq) and HCO3(aq). Alkali metal cations like Na+ do not react with H2O, but HCO3 reacts with H2O to form OH. Thus, NaHCO3, a salt derived from a strong base and a weak acid, forms a basic solution.

When NH4Cl dissolves in water it forms NH4+(aq) and Cl(aq). Halide anions like Cl do not react with H2O, but NH4+ reacts with H2O to form H3O+. Thus, NH4Cl, a salt derived from a strong acid and a weak base, forms an acidic solution.

When NaCl dissolves in water it forms Na+(aq) and Cl(aq). Neither ion reacts with water. Since no acid–base reaction occurs, the solution remains neutral. Thus, NaCl, a salt derived from a strong acid and strong base, forms a neutral solution. The acidity and basicity of salt solutions is summarized in Table 9.4.

TABLE 9.4
The Acidity and Basicity of Salt Solutions
Cation Derived from Anion Derived from Solution pH Examples
Strong base Strong acid neutral 7 NaCl, KBr, NaNO3
Strong base Weak acid basic > 7 NaHCO3, KCN, CaF2
Weak base Strong acid acidic < 7 NH4Cl, NH4NO3
SAMPLE PROBLEM 9.17

Determine whether each salt forms an acidic, basic, or neutral solution when dissolved in water: (a) NaF; (b) KNO3; (c) NH4Br.

ANALYSIS

Determine what type of acid and base (strong or weak) are used to form the salt. When the ions in the salt come from a strong acid and strong base, the solution is neutral. When the ions come from acids and bases of different strength, the ion derived from the stronger reactant determines the acidity.

SOLUTION
PROBLEM 9.31

Determine whether each salt forms an acidic, basic, or neutral solution when dissolved in water: (a) KI; (b) K2CO3; (c) Ca(NO3)2; (d) NH4I; (e) BaCl2; (f) Na3PO4.

PROBLEM 9.32

Which of the following salts forms an aqueous solution that has a pH > 7: (a) LiCl; (b) K2CO3; (c) NH4Br; (d) MgCO3?