Equilibria Involving Weak Acids and Bases

The reactions of weak acids and bases with water do not go to completion. So, to calculate the pH of their solutions, we use or K, and the laws of chemical equilibrium. The calculations follow the pattern of Example 14.10 for gas equilibria. In that case the initial gas-phase pressures P° are known, and we calculate the pressures of products resulting from the incomplete reaction. Here we know the initial concentration of acid or base, and calculate the concentrations of products resulting from its partial reaction with water. 

A weak acid has a smaller than 1. Values of the pfC start at zero for the strongest weak acid and range upward. (If the pfC is greater than 14, the compound is ineffective as an acid in aqueous solution.) When a weak acid is dissolved in water, the original concentration is almost always known but partial reaction with water consumes some HA and generates A and H30  

To calculate the amounts of H30, A, and HA at equilibrium, we use the methods of Chapter 14, with partial pressures replaced by concentrations. A new feature here is that one of the products (H30 ) can also come from a second source, the autoionization of the solvent, water. In most of the applications we study, this second effect is small and can be neglected in the equations. Even so, it is a good idea to verify at the end of each calculation that the [H30 ] from the acid ionization alone exceeds 10 M by at least one order of magnitude. Otherwise, we have to use the more complete method of analysis given in Section 15.8. 

Acetic acid (CH3COOH) has a of 1.76 X 10 at 25°C. Suppose 1.000 mol is dissolved in enough water to give 1.000 L of solution. Calculate the pH and the fraction of acetic acid ionized at equilibrium. 

The initial concentration of acetic acid is 1.000 M. If y mol ionizes, then 

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