Weak Acids and Acid Ionization Constants

Strategy Use Equation 16.6 to convert from pH to pOH and Equation 16.5 to determine the hydroxide ion concentration. Consider the stoichiometiy of dissociation in each case to determine the concentration of the base itself. 

Remember to keep an additional significant figure or two until the end of the problem—to avoid rot/nd/ng error [W Section 1.5]. 

Think About It Alternatively, we could determine the hydroxide ion concentration using Equation 16.3, [HjO ] = 10 = 7.1 X 10 M and Equation 16.1, 

Once [OH ] is known, the solution is the same as shown previously. 

Practice Problem A An aqueous solution of a strong base has pH 8.98 at 25°C. Calculate the concentration of base in the solution (a) if the base is LiOH and (b) if the base is Ba(OH)2. 

As we have seen, there are relatively few strong adds. The vast imgority of acids are weak acids. Consider a weak monoprotic add, HA. Its ionizalion in water is represented by 

Generally, we can calculate the hydrogen ion concentration or pH of an acid solution at equilibrium, given the initial concentration of the acid and its A a value. 

The back endpaper gives an index to all the useful tables and figures in this text. 

TABLE 15.3 Ionization Constants of Some Weak Acids and Their Conjugate Bases at 25°C 

Alternatively, if we know the pH of a weak acid solution and its initial concentration, we can determine its A a- The basic approach for solving these problems, which deal with equilibrium concentrations, is the same one outlined in Chapter 14. However, because acid ionization represents a major category of chemical equilibrium in aqueous solution, we will develop a systematic procedure for solving this type of problem that will also help us to understand the chemistry involved. 

Most acids are weak acids, which ionize only to a limited extent in water. At equilibrium, an aqueous solution of a weak acid contains a mixture of aqueous acid molecules, hydronium ions, and the corresponding conjugate base. The degree to which a weak acid ionizes depends on the concentration of the acid and the equilibrium constant for the ionization. 

Consider a weak monoprotic acid HA. Its ionization in water is represented by 

We constract an equilibrium table and enter the starting concentrations of all species in the equilibrium expression. 

Using the reaction stoichiometiy, we determine the changes in all species  

Finally, we express the equilibrium concentration of each species in terms of x.  

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WEAK ACIDS AND ACID IONIZATION CONSTANTS Review Questions... 

SECTION 16.5 Weak Acids and Acid Ionization Constants... 

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