Discussion
The acid which has the highest ionization constant (Ka) at 298 K is
Answer: E
The key gives HF, the strongest of the five acids listed, but the textbook tables do not agree on its margin over nitrous acid.
The other three are far weaker: phenol has \(K_a\) near \(10^{-10}\), hypochlorous acid near \(10^{-8}\), and a pyridinecarboxylic acid, \(\mathrm{C_5H_4NCOOH}\), only of the order of \(10^{-5}\). That leaves HF and \(\mathrm{HNO_2}\), both near \(10^{-4}\).
General chemistry reference tables at 298 K give HF about \(6.4 \times 10^{-4}\) and \(\mathrm{HNO_2}\) about \(4.6 \times 10^{-4}\), so HF is the stronger and option E matches the key. The NCERT table lists HF as \(3.5 \times 10^{-4}\) and \(\mathrm{HNO_2}\) as \(4.5 \times 10^{-4}\), which would put nitrous acid (option A) first. The question rests on which table is meant; the official key follows the first.
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