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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ie110

Equal volumes of $0.1 M$ $HCl$ and $0.1 M$ $HA$ ( $K_{a} = 1.0 \times 1 0^{- 5}$ ) are mixed at 298 K (use $lo g 5 = 0.699$ ). Select the correct statement(s).

A $HA$ is about $1%$ ionised in the mixture.
BThe degree of ionisation of $HA$ in the mixture is about $2 \times 1 0^{- 4}$ .
C $[A X^{-}] \approx 1 \times 1 0^{- 5} M$ in the mixture.
DThe pH of the mixture is about 1.30.

Answer: B, C, D

After mixing

[HCl] = [HA] = 0.05 M

, so

[H X^{+}] \approx 0.05

and

pH = 2 - 0.699 = 1.30

.

α = K_{a} / [H X^{+}] = 1 0^{- 5} /0.05 = 2 \times 1 0^{- 4}

, hence

[A X^{-}] = 0.05 \times 2 \times 1 0^{- 4} = 1 \times 1 0^{- 5} M

. The

1%

figure is the value for

HA

alone at

0.1 M

.

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