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Among the following compounds, which is the most acidic?

1. p-nitrophenol
2. p-hydroxybenzoic acid
3. o-hydroxybenzoic acid
4. p-toluic acid

Correct answer: o-hydroxybenzoic acid

Solution

o-Hydroxybenzoic acid (salicylic acid) is the most acidic because the ortho -OH group provides intramolecular hydrogen bonding that stabilises the carboxylate anion, giving it a pKa of about 2.98, the lowest among the four options.

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