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JEE Main levelSingle correct optionDifficulty 2 of 3ie024

A $0.2 M$ solution of a weak monobasic acid HA has pH $3.00$ . Neglecting the amount ionised compared with $0.2 M$ , $K_{a}$ of the acid is

A $1 \times 1 0^{- 6}$
B $2.5 \times 1 0^{- 5}$
C $5 \times 1 0^{- 3}$
D $5 \times 1 0^{- 6}$

Answer: D

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

, so

K_{a} = [H X^{+}]^{2} / c = 1 0^{- 6} /0.2 = 5 \times 1 0^{- 6}

. Using

1 0^{- 6}

alone forgets to divide by

c

.

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