Calculate Ph 0.136 M Hc5h9o2 and 0.136 M Nac5h9o2

Introduction

The pH of a solution is a measure of its acidity or basicity. For weak acids and their salts, the pH depends on the concentration of the species present in solution. Pentanoic acid (HC5H9O2) is a weak acid, while sodium pentanoate (NaC5H9O2) is its conjugate base.

When calculating the pH of these solutions, we need to consider the dissociation constants and the concentrations of the acid and its conjugate base. The pH of a solution containing both a weak acid and its conjugate base can be determined using the Henderson-Hasselbalch equation.

pH Calculation Formula

The pH of a solution containing a weak acid (HA) and its conjugate base (A⁻) can be calculated using the Henderson-Hasselbalch equation:

Where:

• pKa is the negative logarithm of the acid dissociation constant (Ka)
• [A⁻] is the concentration of the conjugate base
• [HA] is the concentration of the weak acid

For pentanoic acid (HC5H9O2), the pKa value is approximately 4.86. For sodium pentanoate (NaC5H9O2), since it's a salt, it dissociates completely in water, providing A⁻ ions.

Worked Example

Let's calculate the pH of a solution containing 0.136 M HC5H9O2 and 0.136 M NaC5H9O2.

1. Identify the concentrations: [HA] = 0.136 M, [A⁻] = 0.136 M
2. Use the pKa value for HC5H9O2: pKa = 4.86
3. Apply the Henderson-Hasselbalch equation:
   pH = 4.86 + log(0.136 / 0.136) = 4.86 + log(1) = 4.86 + 0 = 4.86
4. The calculated pH is 4.86, which matches the pKa of the weak acid.

This result makes sense because when the concentrations of the acid and its conjugate base are equal, the pH equals the pKa of the weak acid.

Frequently Asked Questions