Ch.17 - Aqueous Ionic Equilibrium

Tro - Chemistry: A Molecular Approach 4th Edition

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Tro 4th Edition

Ch.17 - Aqueous Ionic Equilibrium

Problem 31

Chapter 17, Problem 31

Calculate the percent ionization of a 0.15 M benzoic acid solution in pure water and in a solution containing 0.10 M sodium benzoate. Why does the percent ionization differ significantly in the two solutions?

Verified step by step guidance

Step 1: Write the chemical equation for the ionization of benzoic acid (C6H5COOH) in water.

Step 2: Use the expression for the acid dissociation constant (Ka) of benzoic acid to set up the equilibrium expression: Ka = [C6H5COO^-][H^+]/[C6H5COOH].

Step 3: For the pure water solution, assume initial concentrations and changes in concentration at equilibrium to solve for [H^+] and calculate percent ionization: % ionization = ([H^+]/[C6H5COOH_initial]) * 100.

Step 4: For the solution containing sodium benzoate, apply the common ion effect. Sodium benzoate provides additional benzoate ions (C6H5COO^-), which affects the equilibrium position. Set up the modified equilibrium expression and solve for [H^+].

Step 5: Calculate the percent ionization in the sodium benzoate solution and compare it to the pure water solution. Explain that the presence of the common ion (C6H5COO^-) suppresses the ionization of benzoic acid, leading to a lower percent ionization.

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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 concentration of the acid and the presence of other species in the solution.

Common Ion Effect

The common ion effect describes how the addition of a salt containing an ion common to a weak acid suppresses its ionization. In the case of benzoic acid and sodium benzoate, the benzoate ion from sodium benzoate shifts the equilibrium, reducing the percent ionization of benzoic acid in the solution.

Percent Ionization Calculation

Percent ionization is calculated by dividing the concentration of ionized acid by the initial concentration of the acid, then multiplying by 100. This metric helps quantify how much of the acid has dissociated into ions, providing insight into the acid's strength and behavior in different environments.

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Calculating Percent Ionization of Weak Acids

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