Water Ionization and the Ion Product Constant

The fact that a small number of water molecules in pure distilled water ionize was mentioned in Section 12.2. This ionization is an equilibrium and can be written as follows  

This equilibrium lies far to the left (indicating that very little ionization has taken place). This means that the concentrations of hydrogen ions and hydroxide ions are very small. At 25°C, [H+] = [OH ] = 1.00 x 10 M. A special equilibrium constant, K, known as the ion product constant for water, is based on this ionization and is defined as follows at 25°C. 

Because it is an equilibrium, the addition of acids or bases to water shifts the position of this equilibrium to the left. The removal of acids or bases through neutralization reactions shifts the position of the equilibrium to the right. The addition of acids increases the [ff], and the resulting shift causes [OH-] to decrease. The addition of bases (hydroxides) increases [OH ], and the resulting shift causes 

knowing either [H+] or [OH ], one can calculate the other from Equation 12.8. We will assume 25°C for all calculations involving 

Much of the chemistry that occurs in water solution depends on what the acid content, or acidity level of the solution is. For example, a procedure for the determination of caffeine in soda pop calls for a solution that has a [H+] of 1.00 X 10 3 M. A procedure for the extraction of potassium from soil requires a solution that has a [H+] of 1.00 x 10 M. Indeed, many activities in our everyday lives depend on the acidity of solutions. Gardeners who grow rhododendrons, for example, should use soil that has a [H+] between about 1 x lO M and 3 x 10 M. The fluid in our stomachs should have [H+] of between 1 X 10 i M and 1 x 10 M. Tropical fish in an aquarium survive only if the [H1 is between 1 x 10 M and 1 x 10 M. Fish in our lakes and rivers have been threatened by acid rain. The [H+] in lakes and rivers should be above 1 X 10 M if fish and other aquatic life are to survive. If the [H+] in our blood is not within a very narrow range (1.00 x 10 -1.58 x 10 M), we die. 

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