http://www.ck12.org Chapter 21. Acids and Bases
21.2 The pH Concept
Lesson Objectives
- Describe the equilibrium process of water ionizing into hydrogen and hydroxide ions, and explain the ion-
product constant. - Calculate the concentration of either the hydrogen ion or the hydroxide ion in water when the other value is
known. - Calculate the pH of an aqueous solution of an acid or a base.
- Calculate the hydrogen-ion concentration of a solution with a known pH.
- Calculate the pOH of acids and bases and know the relationships between pH, pOH, hydrogen-ion concentra-
tion, and hydroxide-ion concentration.
Lesson Vocabulary
- acidic solution
- basic solution
- ion-product of water
- pH
- pOH
- self-ionization
Check Your Understanding
Recalling Prior Knowledge
- What ion is present in an acidic solution according to the Arrhenius theory?
- What ion is present in a basic solution according to the Arrhenius theory?
The acidity or basicity of a solution is a very important property. Natural bodies of water like lakes and streams have
many dissolved components. The acidity of the water is very important to the fish and other organisms that live in
the water. The concept of pH is used by chemists to keep track of how acidic or basic aqueous solutions are, and
adjusting the pH is important in many practical applications. In this lesson, you will learn the meaning of pH and
how to calculate it.
Self-Ionization of Water
Water is a molecular compound, so you may not necessarily expect it to break apart into ions. However, sensitive
experiments show that water is actually a very weak electrolyte. When two molecules of water collide, there can be