4. Calculate The Ph of 0.15 M Acetic Acid.

This guide explains how to calculate the pH of 0.15 M acetic acid solution using the Henderson-Hasselbalch equation. We'll cover the formula, assumptions, and practical applications of this calculation in chemistry.

How to Calculate the pH of Acetic Acid

The pH of an acetic acid solution can be determined using the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the ratio of the concentrations of a weak acid and its conjugate base.

For acetic acid (CH₃COOH), which is a weak acid, the pH is calculated based on its dissociation constant (Ka) and the concentration of the acid. The key steps are:

Determine the dissociation constant (Ka) of acetic acid
Calculate the pKa value from the Ka
Apply the Henderson-Hasselbalch equation
Interpret the resulting pH value

Note: This calculation assumes the solution is a pure acetic acid solution without any added base or buffer components.

The Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution. For acetic acid, the equation is:

pH = pKa + log₁₀([A⁻]/[HA])

Where:

pH is the negative logarithm of the hydrogen ion concentration
pKa is the negative logarithm of the acid dissociation constant
[A⁻] is the concentration of the conjugate base (acetate ion)
[HA] is the concentration of the weak acid (acetic acid)

The dissociation constant (Ka) for acetic acid is approximately 1.8 × 10⁻⁵ at 25°C. The pKa is then calculated as:

pKa = -log₁₀(Ka) = -log₁₀(1.8 × 10⁻⁵) ≈ 4.74

Worked Example

Let's calculate the pH of a 0.15 M acetic acid solution:

Given: [HA] = 0.15 M (acetic acid concentration)
Assume [A⁻] = 0 M (no conjugate base present)
pKa = 4.74 (from above)
Apply the Henderson-Hasselbalch equation:
pH = 4.74 + log₁₀(0/0.15) = 4.74 + log₁₀(0) = 4.74 - ∞ = -∞
Interpretation: A pure acetic acid solution without any conjugate base will have a pH much lower than 0, indicating it's a strong acid solution.

In reality, pure acetic acid solutions will have a pH of about 2.4 due to the presence of small amounts of water dissociation products.

Frequently Asked Questions

What is the pH of 0.15 M acetic acid?

The pH of a 0.15 M acetic acid solution is approximately 2.4, not the calculated -∞ value. This is because pure acetic acid solutions contain small amounts of water dissociation products that affect the pH.

Why does the Henderson-Hasselbalch equation give a negative pH for pure acetic acid?

The equation gives a negative pH because it assumes no conjugate base is present. In reality, water dissociation provides a small amount of OH⁻ ions that buffer the solution to a pH of about 2.4.

What factors affect the pH of acetic acid solutions?

Factors include the concentration of acetic acid, the presence of conjugate base (acetate ions), temperature, and the presence of other solutes that may affect the dissociation equilibrium.