Calculate Ph of 0.2500 M Solution of Ch3coona

Introduction

Sodium acetate (CH3COONa) is a salt formed by the neutralization of acetic acid (CH3COOH) with sodium hydroxide (NaOH). In aqueous solution, it dissociates into acetate ions (CH3COO⁻) and sodium ions (Na⁺). The acetate ion is the conjugate base of acetic acid, which is a weak acid.

The pH of a solution containing a weak acid and its conjugate base can be calculated using the Henderson-Hasselbalch equation, which relates the pH to the acid dissociation constant (Ka) and the ratio of the concentrations of the conjugate base to the weak acid.

How to Calculate the pH of a 0.2500 M Solution of CH3COONa

The pH of a solution of CH3COONa can be calculated using the following steps:

1. Identify the concentration of the salt (CH3COONa) in the solution. In this case, it is 0.2500 M.
2. Determine the acid dissociation constant (Ka) of acetic acid (CH3COOH). The Ka of acetic acid is approximately 1.8 × 10⁻⁵ at 25°C.
3. Use the Henderson-Hasselbalch equation to calculate the pH:
   pH = pKa + log₁₀([CH3COO⁻]/[CH3COOH])
4. Since CH3COONa is a salt, it dissociates completely in solution, providing CH3COO⁻ ions. The concentration of CH3COO⁻ is equal to the concentration of CH3COONa, which is 0.2500 M.
5. Assume that the solution is at equilibrium, and the concentration of CH3COOH is negligible compared to CH3COO⁻. Therefore, the ratio [CH3COO⁻]/[CH3COOH] is very large, and the log term approaches infinity. This means the pH is determined primarily by the pKa of acetic acid.

Example Calculation

Let's calculate the pH of a 0.2500 M solution of CH3COONa:

1. Given:
   • Concentration of CH3COONa (and thus CH3COO⁻) = 0.2500 M
   • Ka of CH3COOH = 1.8 × 10⁻⁵
   • pKa = -log(Ka) = -log(1.8 × 10⁻⁵) ≈ 4.74
2. Using the Henderson-Hasselbalch equation:
   pH = pKa + log₁₀([CH3COO⁻]/[CH3COOH])
3. Since [CH3COOH] is negligible compared to [CH3COO⁻], the log term approaches infinity, and the pH is determined by the pKa of acetic acid.
4. Therefore, pH ≈ pKa ≈ 4.74

The calculated pH of the 0.2500 M solution of CH3COONa is approximately 4.74.

Interpretation of Results

A pH of 4.74 indicates that the solution is slightly acidic. This is expected because the acetate ion (CH3COO⁻) is the conjugate base of acetic acid (CH3COOH), and the solution contains more of the conjugate base than the weak acid. The pH is slightly higher than the pKa of acetic acid (4.74) because the concentration of the conjugate base is greater than the concentration of the weak acid.

This type of solution is commonly used as a buffer because it can resist changes in pH when small amounts of acid or base are added. The buffer capacity is highest when the concentrations of the weak acid and its conjugate base are equal, which is not the case here but is a useful concept for understanding buffer solutions.