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JEE Main levelSingle correct optionDifficulty 2 of 3g2077

Arrange in decreasing order of basic strength (pKa of conjugate acids: methane about 48, ammonia 38, water 15.7, HF 3.2): $CH X_{3} X^{-}$ , $NH X_{2} X^{-}$ , $OH X^{-}$ , $F X^{-}$ .

A $F X^{-}$ > $OH X^{-}$ > $NH X_{2} X^{-}$ > $CH X_{3} X^{-}$
B $NH X_{2} X^{-}$ > $CH X_{3} X^{-}$ > $OH X^{-}$ > $F X^{-}$
C $CH X_{3} X^{-}$ > $NH X_{2} X^{-}$ > $F X^{-}$ > $OH X^{-}$
D $CH X_{3} X^{-}$ > $NH X_{2} X^{-}$ > $OH X^{-}$ > $F X^{-}$

Answer: D

Along the second period the acidity of H-X increases with electronegativity of X, so the basic strength of the conjugate bases falls:

CH X_{3} X^{-} > NH X_{2} X^{-} > OH X^{-} > F X^{-}

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