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JEE Main levelSingle correct optionDifficulty 2 of 3ie032

For ammonia $p K_{b} = 4.75$ at 298 K. The $p K_{a}$ of its conjugate acid $NH X_{4} X^{+}$ is

A $4.75$
B $18.75$
C $2.25$
D $9.25$

Answer: D

For a conjugate pair

K_{a} K_{b} = K_{w}

, so

p K_{a} + p K_{b} = 14

and

p K_{a} = 14 - 4.75 = 9.25

.

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