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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ie107

Two separate $0.1 M$ solutions of weak monobasic acids $HA$ ( $K_{a} = 1 0^{- 5}$ ) and $HB$ ( $K_{a} = 1 0^{- 7}$ ) are prepared at 298 K. Select the correct statement(s). (Use $α ≪ 1$ .)

AThe pH of $HA$ is lower than that of $HB$ by 1 unit.
B $[A X^{-}] / [B X^{-}] = 10$ in the two separate solutions.
CThe degree of ionisation of $HA$ is 10 times that of $HB$ .
DEqual volumes of the two solutions mixed give pH 3.5.

Answer: A, B, C

HA:

[H X^{+}] = 1 0^{- 3}

, pH 3,

α = 0.01

. HB:

[H X^{+}] = 1 0^{- 4}

, pH 4,

α = 0.001

. So the pH differs by 1,

α

ratio is 10, and

[A X^{-}] / [B X^{-}] = 1 0^{- 3} /1 0^{- 4} = 10

. On mixing (0.05 M each)

[H X^{+}] = 0.05 \times 1 0^{- 5} + 0.05 \times 1 0^{- 7} = 7.11 e - 04

, pH

3.15

, not 3.5.

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