Between orthonitrophenol and paranitro phenol which one has lower $P^{k a}$ value.

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Solution

$p K_{a}$ value of an acid determines whether the acid is strong or weak. A strong acid has low $p K_{a}$ and high $K_{a}$ value. While a weak acid has high $p K_{a}$ and low $K_{a}$ value.
Since, the $- I$ effect of nitro group $(- N O_{2})$ is more at para position than at ortho position. Therefore, in para nitrophenol due to stronger $- I$ effect of $- N O_{2}$ the electron pair on $O$ of $- O H$ group will be attracted strongly towards the ring and the $O - H$ bond will become weak leading to the release of $H^{+}$ ion. Thus, making it strongly acidic.
In ortho nitrophenol the $- N O_{2}$ group is present at ortho position and the $- I$ effect is less in comparison to para position. Here the release of $H^{+}$ ion is not as fast as in para.
Thus, para nitrophenol being more acidic has lower $p K_{a}$ value.

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