Exact and Approximate Calculations of Acid-Base Equilibria
In this Demonstration, we show exact and approximate calculations for acid-base equilibria.[more]
The equation for acid dissociation (and similarly for base) is given by , where is the acid (base) concentration and is the acid (base) dissociation constant. This equation can be approximated in the case by . The first graphic shows the results for concentration and the dissociation calculated both exactly and approximately. The appropriate number of significant digits is used.
In the case of a strong acid/base concentration, the water dissociation is determined by , where is the acid (base) concentration and is the autoionization constant of water (at , ). When , this can be approximated by . Use the checkbox to switch from acid to base.[less]
Snapshot 1: Exact and approximate equations give the same result since the initial concentration is high and the extent of dissociation is very low (weak acid).
Snapshot 2: Exact and approximate equations are very different since we have a very high acid dissociation constant. In this case, the acid is completely dissociated (strong acid).
Snapshot 3: Incorrect use of the approximate equation gives the pH of a base although the solution is a slightly acidic.
 D. C. Harris, Quantitative Chemical Analysis, 8th ed., New York: W. H. Freeman and Company, 2010.