Benzoic Acid Is Calculate Ph for 0.25m Denzoic Acid

Introduction

Benzoic acid (C₇H₆O₂) is a weak monoprotic acid with a pKa of approximately 4.20. When dissolved in water, it dissociates partially, forming benzoate ions (C₇H₅O₂⁻) and hydrogen ions (H⁺). The pH of the solution depends on the concentration of benzoic acid and its conjugate base.

For a 0.25M benzoic acid solution, we can calculate the pH using the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the pKa of the acid and the ratio of the conjugate base to the acid.

Formula

For a pure 0.25M benzoic acid solution, the concentration of benzoate ions is initially zero because no dissociation has occurred. As the solution approaches equilibrium, some benzoic acid dissociates, creating equal concentrations of benzoic acid and benzoate ions.

Calculation

For a 0.25M benzoic acid solution at equilibrium:

• Initial concentration of benzoic acid = 0.25M
• At equilibrium, [C₇H₅O₂⁻] = [C₇H₆O₂] = x
• Using the equilibrium expression: Kₐ = [H⁺][C₇H₅O₂⁻]/[C₇H₆O₂]
• For benzoic acid, Kₐ ≈ 6.3 × 10⁻⁵

Solving the equilibrium equation gives x ≈ 0.00025M. Therefore, the pH can be calculated as:

This shows that a 0.25M benzoic acid solution has a pH of approximately 4.20, which is characteristic of a weak acid solution.

Interpretation

The calculated pH of 4.20 indicates that the solution is acidic, as expected for benzoic acid. The pH is slightly lower than the pKa because the solution is not perfectly buffered - some undissociated benzoic acid remains.

Adding a base to this solution would increase the pH, as more benzoic acid would dissociate to form benzoate ions. Similarly, adding an acid would decrease the pH, as more benzoate ions would combine with hydrogen ions to form benzoic acid.