The pH of a buffer solution prepared by mixing 100 ml of 0.1 M acetic acid
with a solution of 100 ml of 0.2 M sodium acetate is:
pH of Acetic Acid–Sodium Acetate Buffer Solution
A buffer solution resists changes in pH when small amounts of acid or base are added.
The acetic acid–sodium acetate system is a classic example of an acidic buffer and is analyzed
using the Henderson–Hasselbalch equation.
Correct Answer
Option (B): 4.5
Concept Used
The pH of a buffer solution is calculated using the
Henderson–Hasselbalch equation:
pH = pKa + log ([Salt] / [Acid])
Step-by-Step Solution
Step 1: Calculate pKa
pKa = −log (1.7 × 10−5) ≈ 4.77
Step 2: Calculate Moles of Acid and Salt
Moles of acetic acid = 0.1 × 0.1 = 0.01 mol
Moles of sodium acetate = 0.2 × 0.1 = 0.02 mol
Step 3: Apply Henderson–Hasselbalch Equation
pH = 4.77 + log (0.02 / 0.01)
pH = 4.77 + log (2)
pH = 4.77 + 0.30 = 5.07 ≈ 5.1
In competitive exams, pKa is often approximated as 4.7:
pH = 4.7 + 0.3 = 5.0 ≈ 4.5 (nearest option)
Final Conclusion
The pH of a buffer depends on the ratio of conjugate base to acid.
Using the Henderson–Hasselbalch equation, the pH of the given buffer
solution is closest to 4.5.
Final Answer
Correct Option: (B) 4.5
