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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ie053

Weak monobasic acids HA ( $p K_{a} = 4.0$ ) and HB ( $p K_{a} = 6.0$ ) are each present as $0.1 M$ solutions (separately) at 298 K, with $α ≪ 1$ . Which statements are correct?

A $[H X^{+}]$ in HA is $10$ times that in HB.
BThe degree of ionisation of HA is $10$ times that of HB.
CThe pH of the HA solution is $2.5$ .
DThe pH values of the two solutions differ by $2$ units.

Answer: A, B, C

[H X^{+}] = K_{a} c

: HA gives

1 0^{- 4} \times 0.1 = 1 0^{- 2.5}

(pH

2.5

) and HB gives

1 0^{- 6} \times 0.1 = 1 0^{- 3.5}

(pH

3.5

). So

[H X^{+}]

and

α

both differ by a factor of

10

(same

c

) and the pH differs by

1

unit, not

2

.

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