MHT CET · Chemistry · Ionic Equilibrium

A buffer solution contains equal concentrations of weak acid and its salt with strong base. Calculate pH of buffer solution if dissociation constant of weak acid is \(1.8 \times 10^{-5}\).

A 4.7447
B 5.142
C 5.8496
D 4.0128

Verified Solution

Answer & Solution

Correct Answer

(A) 4.7447

Step-by-step Solution

Detailed explanation

For an acidic buffer,
\(\therefore \mathrm{pH}=\mathrm{pK}_{\mathrm{a}}+\log \frac{[\text { Salt }]}{[\text { Acid }]} \)
\( (\because[\text { Salt }]=[\text { Acid }]) \)
\(\therefore \mathrm{pH}=\mathrm{pK}_{\mathrm{a}}=-\log \mathrm{K}_{\mathrm{a}}=-\log \left(1.8 \times 10^{-5}\right) \)
\(\therefore \mathrm{pH}=-[\log 1.8-5 \log 10]=4.7447\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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