Calculate Ph of 0.2 M Aqueous Solution of Sodium Butyrate

Sodium butyrate (C₄H₇NaO₂) is a short-chain fatty acid salt that dissolves in water to form a buffer solution. Calculating the pH of a 0.2 M aqueous solution of sodium butyrate requires understanding the dissociation of the weak acid and its conjugate base.

Introduction

Sodium butyrate is a weak acid salt that dissociates in water according to the following equilibrium:

C₄H₇NaO₂ ⇌ C₄H₇O₂⁻ + Na⁺

The pH of the solution depends on the concentration of the butyric acid (C₄H₇O₂) and its conjugate base (C₄H₇O₂⁻). For a 0.2 M solution, we can calculate the pH using the Henderson-Hasselbalch equation.

Calculation Method

The pH of a weak acid solution can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([C₄H₇O₂⁻]/[C₄H₇O₂])

Where:

pKa is the acid dissociation constant of butyric acid (-log(Ka))
[C₄H₇O₂⁻] is the concentration of the conjugate base
[C₄H₇O₂] is the concentration of the undissociated acid

For a 0.2 M solution of sodium butyrate, we assume complete dissociation, so [C₄H₇O₂⁻] = 0.2 M and [C₄H₇O₂] = 0.2 M.

Example Calculation

Using the pKa value for butyric acid (4.82) and the given concentrations:

pH = 4.82 + log(0.2/0.2) = 4.82 + log(1) = 4.82 + 0 = 4.82

Therefore, the pH of a 0.2 M aqueous solution of sodium butyrate is 4.82.

Interpretation

A pH of 4.82 indicates that the solution is slightly acidic, which is expected for a weak acid solution. This pH is typical for short-chain fatty acid solutions and is within the range where many biological processes occur.

Note: The actual pH may vary slightly due to impurities or temperature effects, but this calculation provides a good approximation.

FAQ

What is the pKa of butyric acid?: The pKa of butyric acid is approximately 4.82 at 25°C.
Does the pH change with concentration?: For a weak acid solution, the pH remains relatively constant over a wide range of concentrations, as shown by the Henderson-Hasselbalch equation.
Is sodium butyrate a strong or weak acid?: Sodium butyrate is a weak acid because it only partially dissociates in water.
Can the pH be calculated for other concentrations?: Yes, the same method can be used for any concentration of sodium butyrate by adjusting the concentrations in the Henderson-Hasselbalch equation.