21.5 Salt Solutions
21.5 Salt Solutions
Lesson Objectives
- Predict whether a certain salt solution is acidic, basic, or neutral.
- Write balanced equations for hydrolysis reactions.
- Calculate the pH of a salt solution when the Kaor the Kbof the hydrolyzing ion is known.
- Use equations to show how buffers resist changes in pH by using up additional hydrogen or hydroxide ions.
Lesson Vocabulary
- buffer
- buffer capacity
- salt hydrolysis
Check Your Understanding
Recalling Prior Knowledge
- How is the strength of an acid or a base related to its Kaor Kb?
- How is the pH of a weak acid or a weak base calculated?
Many salts are soluble and dissociate completely. The ions produced from the dissociation of a salt are sometimes
capable of undergoing reactions with water that change the pH of the solution. In this lesson, you will learn about
hydrolysis reactions and about an important class of solutions called buffers.
Hydrolysis of Salts
A salt is an ionic compound that is formed when an acid and a base neutralize each other. While it may seem that
salt solutions would always be neutral, they can frequently be either acidic or basic.
Consider the salt formed when the weak acid hydrofluoric acid is neutralized by the strong base sodium hydroxide.
The molecular and net ionic equations are shown below.
HF(aq)+NaOH(aq)→NaF(aq)+H 2 O(l)
HF(aq)+OH−(aq)→F−(aq)+H 2 O(l)Since sodium fluoride is soluble, the sodium ion is a spectator ion in the neutralization reaction. The fluoride ion is
capable of reacting to a small extent with water, accepting a proton.