Suppose you have a sample of benzoic acid dissolved in water.
c. Finally, assume that aqueous HCl is added to the solution from (b) until pH 2 is reached. Draw the structure of the major organic species present.

Verified step by step guidance

1. Identify the chemical structure of benzoic acid (C₆H₅COOH). It is a weak acid with a carboxylic acid functional group (-COOH) that can donate a proton (H⁺) in aqueous solution.
2. Recall that in water, benzoic acid can partially dissociate into its conjugate base, the benzoate ion (C₆H₅COO⁻), and a proton (H⁺). This equilibrium depends on the pH of the solution.
3. Understand that adding aqueous HCl to the solution introduces a strong acid, which significantly increases the concentration of H⁺ ions in the solution, lowering the pH to 2.
4. At pH 2, the solution is highly acidic, and the excess H⁺ ions will shift the equilibrium of benzoic acid dissociation back toward the undissociated form (C₆H₅COOH), according to Le Chatelier's principle.
5. Conclude that the major organic species present at pH 2 will be the undissociated form of benzoic acid (C₆H₅COOH), as the high concentration of H⁺ suppresses the formation of the benzoate ion (C₆H₅COO⁻).

Key Concepts

Benzoic Acid Structure

Benzoic acid is a simple aromatic carboxylic acid with the formula C7H6O2. It consists of a benzene ring attached to a carboxyl group (-COOH). Understanding its structure is crucial for predicting its behavior in solution, particularly how it interacts with acids and bases.

pH and Acid-Base Chemistry

pH is a measure of the acidity or basicity of a solution, with lower values indicating higher acidity. When HCl is added to a solution, it dissociates to release H+ ions, lowering the pH. This process affects the ionization state of benzoic acid, influencing the predominant species in the solution.

Ionization of Carboxylic Acids

Carboxylic acids, like benzoic acid, can donate protons (H+) to form their conjugate base, the carboxylate ion. At low pH, such as pH 2, the majority of benzoic acid remains in its protonated form (non-ionized), which is essential for determining the major organic species present in the solution after HCl addition.