Benzoic Acid Is Calculate Ph for 0.25m Denzoic Acid

Benzoic acid is a weak organic acid commonly used in food preservation and pharmaceuticals. Calculating its pH in solution helps chemists understand its acidity and buffering capacity. This guide explains how to calculate the pH of a 0.25M benzoic acid solution using the Henderson-Hasselbalch equation.

Introduction

The pH of a benzoic acid solution depends on its concentration and the dissociation constant (Ka) of benzoic acid. The Henderson-Hasselbalch equation relates these factors to calculate the pH.

Benzoic acid (C₇H₆O₂) is a weak acid with a pKa of approximately 4.20 at 25°C. This means it partially dissociates in water, forming benzoate ions (C₇H₅O₂⁻) and hydrogen ions (H⁺).

pH Calculation Formula

The Henderson-Hasselbalch equation for a weak acid is:

pH = pKa + log₁₀([A⁻]/[HA])

Where:

pH = negative logarithm of hydrogen ion concentration
pKa = negative logarithm of the acid dissociation constant
[A⁻] = concentration of the conjugate base (benzoate ion)
[HA] = concentration of the weak acid (benzoic acid)

For a 0.25M benzoic acid solution, we assume the concentration of benzoic acid equals the concentration of benzoate ions because the solution is at equilibrium.

Step-by-Step Calculation

Identify the concentration of benzoic acid: 0.25M
Determine the pKa of benzoic acid: 4.20
Assume [A⁻] = [HA] = 0.25M (for a neutral solution)
Calculate the ratio [A⁻]/[HA] = 1
Apply the Henderson-Hasselbalch equation:

pH = 4.20 + log₁₀(1) = 4.20 + 0 = 4.20

The pH of a 0.25M benzoic acid solution is 4.20, which is characteristic of a weak acid solution.

Worked Example

Let's calculate the pH of a 0.25M benzoic acid solution:

Example Calculation

Given:

Concentration of benzoic acid (HA) = 0.25M
pKa of benzoic acid = 4.20
Assuming [A⁻] = [HA] = 0.25M (neutral solution)

Using the Henderson-Hasselbalch equation:

pH = pKa + log₁₀([A⁻]/[HA]) = 4.20 + log₁₀(1) = 4.20

Result: The pH of the solution is 4.20.

This example shows that a 0.25M benzoic acid solution has a pH of 4.20, indicating it is slightly acidic.

Frequently Asked Questions

What is the pKa of benzoic acid?

The pKa of benzoic acid is approximately 4.20 at 25°C. This value indicates the acid's strength and helps calculate its pH in solution.

How do I calculate the pH of a benzoic acid solution?

Use the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]). For a 0.25M solution, assume [A⁻] = [HA] = 0.25M to get pH = 4.20.

Why is benzoic acid used in food preservation?

Benzoic acid has antimicrobial properties and is effective at low concentrations, making it a common preservative in foods and beverages.