Q.36 You have prepared
1.0 liter of
0.5 M acetate buffer
(pH = 5.0).
The dissociation constant of acetic acid is
1.7 × 10−5 M.
What would be the acetate ion concentration in the buffer?
Introduction
Buffer calculations are frequently asked in competitive examinations such as CSIR-NET, GATE, and JAM.
This problem focuses on determining the acetate ion concentration in an acetate buffer
when pH, total buffer concentration, and dissociation constant are provided.
Given Data
- Total buffer concentration = 0.5 M
- Volume of buffer = 1.0 L
- pH of buffer = 5.0
- Dissociation constant of acetic acid,
Ka = 1.7 × 10−5
Step 1: Henderson–Hasselbalch Equation
pH = pKa + log
( [A−] / [HA] )
Step 2: Calculate pKa
pKa = −log(1.7 × 10−5) ≈ 4.77
Step 3: Substitute Values
5.0 = 4.77 + log
( [A−] / [HA] )
log
( [A−] / [HA] ) = 0.23
[A−] / [HA] = 100.23 ≈ 1.7
Step 4: Use Total Buffer Concentration
[A−] + [HA] = 0.5
[A−] = 1.7 [HA]
1.7[HA] + [HA] = 0.5
2.7[HA] = 0.5
[HA] ≈ 0.185 M
[A−] = 1.7 × 0.185 ≈ 0.315 M
Correct Answer
Option (C): 0.315 M
Conclusion
By applying the Henderson–Hasselbalch equation and using the total buffer concentration,
the acetate ion concentration in a 0.5 M acetate buffer at pH 5.0 is found to be
0.315 M. Understanding this method helps solve a wide range of buffer-related
numerical problems in exams.
