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?

Verified step by step guidance

1. insert step 1: Write the chemical equation for the ionization of benzoic acid (C6H5COOH) in water: C6H5COOH (aq) ⇌ C6H5COO⁻ (aq) + H⁺ (aq).
2. insert step 2: Use the expression for the acid dissociation constant (Ka) for benzoic acid: Ka = [C6H5COO⁻][H⁺] / [C6H5COOH].
3. insert step 3: For the pure water solution, set up an ICE table (Initial, Change, Equilibrium) to find the equilibrium concentrations of the species involved. Assume initial concentration of benzoic acid is 0.20 M and initial concentrations of C6H5COO⁻ and H⁺ are 0.
4. insert step 4: For the solution containing sodium benzoate, recognize that sodium benzoate is a salt that provides a common ion (C6H5COO⁻). Use the common ion effect to adjust the ICE table, starting with an initial concentration of 0.25 M for C6H5COO⁻.
5. insert step 5: Calculate the percent ionization for both scenarios using the formula: Percent Ionization = ([H⁺] / Initial [C6H5COOH]) * 100%. Compare the results to understand the effect of the common ion on ionization.

Key Concepts

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.

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.

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.