Calculate The Ph of 0.1 M Acetic Acid

Introduction

The pH of a solution is a measure of its acidity or alkalinity, defined as the negative logarithm of the hydrogen ion concentration (H⁺). For weak acids like acetic acid (CH₃COOH), the pH can be calculated using the Henderson-Hasselbalch equation when the concentration of the conjugate base (acetate ion, CH₃COO⁻) is known.

Acetic acid is a common weak acid with a dissociation constant (Ka) of 1.8 × 10⁻⁵ at 25°C. When dissolved in water, it partially dissociates into acetate ions and hydrogen ions, creating a buffer solution.

How to Calculate pH

To calculate the pH of 0.1 M acetic acid, follow these steps:

1. Determine the concentration of acetic acid (CH₃COOH) and its conjugate base (CH₃COO⁻).
2. Use the Henderson-Hasselbalch equation: pH = pKa + log([CH₃COO⁻]/[CH₃COOH]).
3. Calculate the pH using the dissociation constant (Ka) of acetic acid.

For a solution of 0.1 M acetic acid, the initial concentration of acetate ions is negligible, so the equation simplifies to pH ≈ pKa ≈ 4.74.

Example Calculation

Let's calculate the pH of 0.1 M acetic acid:

1. Given: [CH₃COOH] = 0.1 M, [CH₃COO⁻] ≈ 0 M (initial condition)
2. pKa = -log(1.8 × 10⁻⁵) ≈ 4.74
3. pH = pKa + log([CH₃COO⁻]/[CH₃COOH]) = 4.74 + log(0/0.1) = 4.74 + (-∞) ≈ 4.74

The pH of 0.1 M acetic acid is approximately 4.74, indicating a weak acid solution.

Interpreting Results

The pH of 0.1 M acetic acid is 4.74, which is characteristic of a weak acid. This means:

• The solution is mildly acidic.
• It has a low concentration of free hydrogen ions.
• It can act as a buffer when mixed with its conjugate base.

To verify this result, you can compare it with the pKa of acetic acid (4.74). The pH should be close to the pKa for a solution where the acid and its conjugate base are present in equal amounts.