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Ch.18 - Aqueous Ionic Equilibrium
Chapter 18, Problem 31

Calculate the percent ionization of a 0.20 M benzoic acid solution in pure water and in a solution containing 0.25 M sodium benzoate. Why does the percent ionization differ significantly in the two solutions?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionization of Weak Acids

Weak acids, like benzoic acid, do not completely dissociate in solution. Instead, they establish an equilibrium between the undissociated acid and its ions. The degree of ionization is influenced by the acid's strength, which is quantified by its acid dissociation constant (Ka). Understanding this equilibrium is crucial for calculating percent ionization.
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Percent Ionization

Percent ionization is a measure of the extent to which an acid dissociates in solution, expressed as the ratio of the concentration of ionized acid to the initial concentration, multiplied by 100. It provides insight into the strength of the acid in different environments, helping to compare how factors like concentration and the presence of other solutes affect ionization.
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Common Ion Effect

The common ion effect occurs when a salt containing an ion in common with a weak acid is added to the solution, shifting the equilibrium position according to Le Chatelier's principle. In the case of benzoic acid and sodium benzoate, the presence of benzoate ions from sodium benzoate suppresses the ionization of benzoic acid, leading to a lower percent ionization in the mixed solution compared to pure water.
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