Which of the following is more acidic?
a) 4-nitrophenol      b0 2-nitrophenol

In ortho and para nitro phenol, there is resonance stabilization of the phenoxide ion and hence both of them are more acidic than meta -nitrophenol.

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In para nitrophenol, there is no H-bonding between phenol acidic H and oxygen of the nitro group . But in ortho nitrophenol, H bonding is there  due to proximity of acidic H and negatively charged oxygen of nitro group on the adjacent carbon. Due to this hydrogen bonding in ortho-nitrophenol becomes slightly less acidic.