10 Calculate The Ph of 0.15 M Acetic Acid
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Acetic acid (CH3COOH) is a weak acid commonly found in vinegar. Calculating its pH helps chemists understand its acidity and behavior in solutions. This guide explains how to determine the pH of 0.15 M acetic acid using the Henderson-Hasselbalch equation.
How to Calculate the pH of Acetic Acid
The pH of a weak acid solution can be calculated using the Henderson-Hasselbalch equation, which relates the pH to the acid dissociation constant (Ka) and the ratio of conjugate base to acid concentrations.
To calculate the pH of 0.15 M acetic acid:
- Identify the concentration of acetic acid (Cacid)
- Determine the concentration of acetate ion (Cbase)
- Find the acid dissociation constant (Ka) for acetic acid
- Apply the Henderson-Hasselbalch equation
For pure acetic acid (where Cbase = 0), the pH is simply the negative logarithm of the square root of the Ka.
The Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation is:
pH = pKa + log10([Cbase]/[Cacid])
Where:
- pKa = -log10(Ka)
- Cbase = concentration of conjugate base (acetate ion)
- Cacid = concentration of acid (acetic acid)
The pKa of acetic acid is approximately 4.76 at 25°C. For pure acetic acid (Cbase = 0), the equation simplifies to:
pH = -log10(√Ka)
Worked Example
Let's calculate the pH of 0.15 M acetic acid:
- Given: Cacid = 0.15 M, Cbase = 0 M (pure acid)
- Ka for acetic acid = 1.8 × 10-5
- Calculate pKa = -log10(1.8 × 10-5) ≈ 4.74
- For pure acid: pH = -log10(√1.8 × 10-5) ≈ 2.57
The calculated pH of 0.15 M acetic acid is approximately 2.57. This indicates a strongly acidic solution.
Frequently Asked Questions
What is the pH of 0.15 M acetic acid?
The pH of 0.15 M acetic acid is approximately 2.57, calculated using the Henderson-Hasselbalch equation.
How does concentration affect acetic acid pH?
For pure acetic acid (no conjugate base), concentration does not affect the pH, which is determined solely by the Ka value. However, if the solution contains acetate ions, the pH will depend on the ratio of acetate to acetic acid.
Why is acetic acid considered a weak acid?
Acetic acid is a weak acid because it only partially dissociates in water, with a relatively high Ka value compared to strong acids.