Weak Bases and Base Ionization Constants

The ionization of weak bases is treated in the same way as the ionization of weak acids. When ammonia dissolves in water, it undergoes the reaction 

Compared with the total concentration of water, very few water molecules are consumed by this reaction, so we can treat [H2O] as a constant. Thus, we can write the base ionization constant (Kt), which is the equilibrium constant for the ionization reaction, as 

In solving problems involving weak bases, we follow the same procedure we used for weak acids. The main difference is that we calculate [OH ] first, rather than [H ]. Example 15.10 shows this approach. 

Strategy The procedure here is similar to the one used for a weak acid (see Example 15.8). From the ionization of ammonia, we see that the major species in solution at equilibrium are NH3, NH4, and OH . The hydrogen ion concentration is very small as we would expect from a basic solution, so it is present as a minor species. As before, we ignore the ionization of water. We make a sketch to keep track of the pertinent species as follows  

Step 1 The major species in an ammonia solution are NH3, NHJ, and OH . We ignore the very small contribution to OH concentration by water. 

Just as most acids are weak, most bases are also weak. The ionization of a weak base is incomplete and is treated in the same way as the ionization of a weak acid. In this section, we will see how the ionization constant for a weak base, K, is related to the pH of an aqueous solution. 

Solving problems involving weak bases requires the same approach we used for weak acids. It is important to remember, though, that solving for jn in a typical weak base problem gives us the hydroxide ion concentration rather than the hydronium ion concentration. 

Sample Problem 16.14 shows how to use Ki, to calculate the pH of a weak base solution. 

Strategy Construct an equilibrium table, and express equilibrium concentrations in terms of the unknown x. Plug these equilibrium concentrations into the equilibrium expression, and solve for x. From the value of x, determine the pH. 

Solution The equilibrium concentrations are substituted into the equilibrium expression to give 

TABLE 16.7 I Ionization Constants of Some Weak Bases at 25X 

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WEAK BASES AND BASE IONIZATION CONSTANTS Review Questions... 

SECTION 16.6 Weak Bases and Base Ionization Constants... 

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