MHT CET · Chemistry · Amines

Which of the following salts turns red litmus blue in its aqueous solution?

A \(\mathrm{NH}_4 \mathrm{NO}_3\)
B \(\mathrm{NH}_4 \mathrm{Cl}\)
C \(\mathrm{NH}_4 \mathrm{CN}\)
D \(\mathrm{NH}_4 \mathrm{~F}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\mathrm{NH}_4 \mathrm{CN}\)

Step-by-step Solution

Detailed explanation

\(\mathrm{NH}_4 \mathrm{CN}\) is the salt of weak acid \(\mathrm{HCN}\) \(\left(\mathrm{K}_{\mathrm{a}}=4.0 \times 10^{-10}\right)\) and weak base \(\mathrm{NH}_4 \mathrm{OH}\) \(\left(\mathrm{K}_{\mathrm{b}}=1.8 \times 10^{-5}\right)\) showing that \(\mathrm{K}_{\mathrm{a}} < \mathrm{K}_{\mathrm{b}}\). If \(\mathrm{K}_{\mathrm{a}} < \mathrm{K}_{\mathrm{b}}\), the aqueous solution of the salt will be basic. Thus, the solution of \(\mathrm{NH}_4 \mathrm{CN}\) is basic and turns red litmus blue.

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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