http://www.ck12.org Chapter 21. Acids and Bases
21.2 The pH Concept
Lesson Objectives
- Describe the self-ionization of water, and calculate the equilibrium constant for this reaction.
- Define pH.
- List the pH values of some common materials.
- Calculate pH when given the concentration of hydrogen ions.
- Calculate the concentration of hydrogen ions when given the pH of a solution.
- Describe ways of measuring the pH of a solution.
Lesson Vocabulary
- amphoteric: A substance that can act as both an acid and a base.
- neutral solution: An aqueous solution in which [H 3 O+] = [OH−].
- acidic solution: An aqueous solution in which [H 3 O+] >[OH−].
- basic solution: An aqueous solution in which [H 3 O+] <[OH−].
- pH: A measure of the concentration of hydrogen atoms in solution;pH=−log[H+].
- acid-base indicator: A substance which changes color based on the relative acidity of the solution.
Check Your Understanding
- Based on what you know about acids and bases, would you expect an acidic solution to have a high or low
concentration of H+ions?
Self-Ionization of Water
Water is an example of what is known as anamphotericsubstance, which means that it can act as both an acid and a
base. In the presence of a strong acid, water can be a proton acceptor (a base), producing the hydronium ion (H 3 O+):
HCl(aq)+H 2 O(l)→Cl−(aq)+H 3 O+(aq)However, water can also donate a proton (acting as an acid) when combined with a strong enough base, producing
the hydroxide ion (OH−):
CH 3 O−(aq)+H 2 O(l) CH 3 OH(aq)+OH−(aq)Overall, water is a weak acid and a weak base. Because it has both of these properties, any sample of liquid water
undergoes the following acid-based reaction in which both hydronium and hydroxide ions are produced to a very
small extent: