Determine the percent ionization of a 0.250 M solution of benzoic acid.

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Identify the chemical formula for benzoic acid, which is C6H5COOH, and recognize that it is a weak acid that partially ionizes in solution.

Write the ionization equation for benzoic acid: C6H5COOH (aq) ⇌ C6H5COO⁻ (aq) + H⁺ (aq).

Use the expression for the acid dissociation constant (Ka) for benzoic acid: Ka = [C6H5COO⁻][H⁺] / [C6H5COOH].

Assume that the initial concentration of benzoic acid is 0.250 M and that the change in concentration due to ionization is 'x', then set up the expression: Ka = (x)(x) / (0.250 - x).

Solve for 'x', which represents the concentration of ionized benzoic acid, and calculate the percent ionization using the formula: (x / 0.250) * 100%.

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Key Concepts

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

Ionization of Acids

Ionization refers to the process by which an acid donates protons (H+) to water, forming hydronium ions (H3O+) and its conjugate base. For weak acids like benzoic acid, this process is not complete, and only a fraction of the acid molecules ionize in solution. Understanding this concept is crucial for calculating the percent ionization.

Percent Ionization

Percent ionization is a measure of the extent to which an acid ionizes in solution, expressed as a percentage. It is calculated using the formula: (concentration of ionized acid / initial concentration of acid) × 100%. This concept helps in comparing the strength of different acids and understanding their behavior in solution.

Equilibrium Constant (Ka)

The acid dissociation constant (Ka) quantifies the strength of a weak acid in solution. It is defined as the ratio of the concentration of the products (ionized form) to the concentration of the reactants (non-ionized form) at equilibrium. Knowing the Ka value for benzoic acid is essential for calculating the degree of ionization and, consequently, the percent ionization.

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Equilibrium Constant K

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Textbook Question

Determine the pH of an HF solution of each concentration. In which cases can you not make the simplifying assumption that x is small? (K_a for HF is 6.8×10^–4.) a. 0.250 M b. 0.0500 M c. 0.0250 M

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