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8.3 Acid/Base Properties of Salt Solutions
Salts are solids (at SATP) composed of cations and anions arranged in a crystalline lattice. When dissolved in water, they dissociate into individual hydrated ions that may or may not affect the pH of the solution.
neutral electrolyte: a salt which does not produce hydronium ions (H3O+) or hydroxide ions in solution
hydrolysis: a reaction of an ion with water to produce an acidic or basic solution
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Salts That Form Neutral SolutionsSalts that consist of cations of a strong base (like Na+ and K+) and the anions of strong acids (like Cl and NO3
) have no effect on the pH.
Table 1
Salt Cation From Strong Base
Anion From Strong Acid
NaCl NaOH HCl
KCl KOH HCl
NaI NaOH HI
NaNO3 NaOH HNO3
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Practice 8.3 (pH of salt solutions)
1. Predict whether the following solutions are acidic, basic, or neutral. Refer to Appendix C9 to assist in the calculations. a) ammonium phosphate b) ammonium sulfatec) sodium sulfite d) ammonium acetate
3. Calculate the pH of each solution: a) 0.30 mol/L ammonium nitrate b) 0.25 mol/L ammonium bromide
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page 827
Make a qualitative prediction, initially, then a more detailed quantitative prediction later for your lab report.
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Salts That Form Basic Solutions
A salt made of a nonhydrolyzing cation (like Na+ or K+) and an anion that is the conjugate base of a weak acid, will form a basic solution.
Example:
Salts That Form Acidic Solutions
Cations that are conjugate acids of weak molecular bases act as weak acids that tend to lower pH.
Table 2Salt Cation (acid) of Weak Base Hydrolysis Reaction
NH4Cl NH4+ NH3
N2H5Br N2H5+ N2H4
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Salts that Act as Acids and Bases
Some salts contain the cation of a weak base and the anion of a weak acid. Both ions can hydrolyze.
Example:
If the Ka is larger than Kb, the solution will be acidic, and vice versa. In this case, the solution will be basic.
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Hydrolysis of Amphoteric Ions
Polyatomic ions whose chemical formulas begin with hydrogen (eg. HCO4
(aq), HSO4
(aq)) are amphoteric. They can accept or
donate a proton.Example:
The following two equilibrium equations are possible:
The solution will be acidic or basic depending which equation has the larger K value (Ka or Kb).Hydrolysis of Metal and Nonmetal Oxides
Metal oxides react with water to produce basic solutions.Nonmetal oxides react with water to produce acidic solutions.
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Assigned WorkA) Read pages 588589 (Amphiprotic Salts)
B) Answer # 6, 7, 8 pages 588, 589 (see p.801 for # 7)
C) Investigation 8.3.1 (see page 827)• choose one obviously acidic salt
choose one obviously basic salt choose one additional salt that is either amphiprotic, or which contains ions which both affect pH.
• Make an initial qualitative prediction for each of the three chosen salts, and complete a more detailed quantitative prediction (ie. exact pH predicted) later for your lab report.
D) Answer # 2, 4 7 page 594,