Acid-dissociation constant percent ionization

M of a weak acid solution (HA) ionizes 0.2 percent in water. Find its acid dissociation constant (Kg) and its pH value ... 

Figure 2.11. Percent of ionogenic (ionizable) species present for weak acids and bases when solution pH is 2 units above or below the acid dissociation constant.

To determine the pH of a solution of a weak acid, you might think that tile percent ionization of the acid would be simpler to use tiian the acid-dissociation constant. However, the percent ionization at a particular temperature depends not only on the identity of the acid but also on its concentration. As shown in Figure 16.9 , the percent ionization of a weak acid decreases as its concentration increases. This fact is further demonstrated in Sample Exercise 16.12. 

This equation essentially describes the relationship between pH and the degree of ionization of weak acids and bases. When applied to drugs, the equation tells us that when pH equals the apparent equilibrium dissociation constant of the drug (pKJ, 50 percent of the drug will be in the unionized form and 50 percent will be in the ionized form (i.e., log[base/acid] = 0 and antilog of 0 = 1, or unity). Application of the Henderson-Hasselbalch equation can, therefore, allow one to mathematically determine the exact proportion of ionized and nonionized species of a drug in a particular body compartment if the pKa of the drug and the pH of the local environment are known. 

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