Chemistry Equilibrium NUMS 2019
PMDC Verified Question 79 of 102
The \( \text{pKa} \) values of \( \text{CH}_3\text{COOH} \) is 4.74, the pH of equimolar solution of acetic acid and sodium acetate is:
A
13.0
B
7.2
C
4.79
D
4.74
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: 4.74
Concept:

The pH of a buffer solution depends on the ratio of the salt (conjugate base) to the weak acid according to the Henderson-Hasselbalch equation.

Formula:

$$ \text{pH} = \text{pK}_a + \log \left( \frac{[\text{Salt}]}{[\text{Acid}]} \right) $$

Solution:

  • The solution is equimolar, meaning \( [\text{Salt}] = [\text{Acid}] \).


  • Therefore, the ratio \( \frac{[\text{Salt}]}{[\text{Acid}]} = 1 \).


  • The logarithm of 1 is zero: \( \log(1) = 0 \).


  • The equation simplifies to \( \text{pH} = \text{pK}_a + 0 \).


  • Since \( \text{pKa} = 4.74 \), the pH is also exactly 4.74.


Why other options are incorrect:

Any deviation from 4.74 would require unequal concentrations of the acid and its salt.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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