why is the solubility of benzoic acid dependent on pH?

Benzoic acid is C6H5COOH. It ionizes as

C6H5COOH ==> H^+ + C6H5COO^-
At low levels of H^+ the reaction shifts to the right; at high levels of H^+ the reaction shifts to the left. Read up on Le Chatelier's Principle.

The solubility of benzoic acid is dependent on pH due to its nature as an acidic compound. Benzoic acid is a weak acid, meaning that it does not completely dissociate in water. Instead, it exists as both a dissolved molecule and its corresponding conjugate base, the benzoate ion. The balance between these two forms is influenced by the pH of the solution.

When benzoic acid is placed in water, it releases a proton (H+) to become the benzoate ion (C6H5COO-). In an acidic environment (low pH), which means a higher concentration of H+ ions, the equilibrium shifts towards the molecular form of benzoic acid. This results in lower solubility because the undissociated molecules are less soluble than the benzoate ions.

On the other hand, in a basic environment (high pH) with a lower concentration of H+ ions, the equilibrium shifts towards the benzoate ion form. The benzoate ions have greater solubility in water compared to the undissociated benzoic acid molecules. Hence, the solubility of benzoic acid increases at higher pH values.

To understand the specific solubility behavior of benzoic acid at different pH levels, it is useful to calculate the acid dissociation constant (Ka) of benzoic acid, which represents the degree of ionization. By knowing the Ka value, one can determine the pH range at which benzoic acid is predominantly in its molecular form or as the benzoate ion. The equation for the dissociation of benzoic acid is as follows:

C6H5COOH ⇌ C6H5COO- + H+

To calculate the solubility at a particular pH, the equilibrium constant (Keq) is used. The formula for Keq is:

Keq = [C6H5COO-] / [C6H5COOH]

In this equation, [C6H5COO-] represents the concentration of benzoate ions, and [C6H5COOH] represents the concentration of benzoic acid. By rearranging the equation, you can solve for the solubility, [C6H5COOH].

Overall, the solubility of benzoic acid is dependent on pH because it is a weak acid that exists in the solution as a balance between its molecular form and the benzoate ion. The pH directly affects this equilibrium by controlling the relative concentrations of these two forms, thereby impacting the solubility behavior of benzoic acid.