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General Chemistry Principles, Patterns, and Applications, 2011

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As shown here for benzoic acid (C6H5CO2H), the percent ionization decreases as the analytical<br />

concentration of a weak acid increases.<br />

Unlike the Ka or the Kb, the percent ionization is not a constant for weak acids <strong>and</strong> bases but depends<br />

on both the Ka or the Kb <strong>and</strong> the analytical concentration. Consequently, the procedure in Example 8 must<br />

be used to calculate the percent ionization <strong>and</strong> pH for solutions of weak acids <strong>and</strong> bases. Example 9 <strong>and</strong><br />

its corresponding exercise demonstrate that the combination of a dilute solution <strong>and</strong> a relatively<br />

large Ka or Kb can give a percent ionization much greater than 5%, making it necessary to use the quadratic<br />

equation to determine the concentrations of species in solution.<br />

Note the Pattern<br />

The percent ionization in a solution of a weak acid or a weak base increases as the analytical<br />

concentration decreases <strong>and</strong> as the Ka or the Kb increases.<br />

E X A M P L E 9<br />

Benzoic acid (C 6H 5CO 2H) is used in the food industry as a preservative <strong>and</strong> medically as an antifungal agent. Its<br />

pK a at 25°C is 4.20, making it a somewhat stronger acid than acetic acid. Calculate the percentage of benzoic<br />

acid molecules that are ionized in each solution.<br />

a. a 0.0500 M solution<br />

b. a 0.00500 M solution<br />

Given: concentrations <strong>and</strong> pK a<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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