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JEE Main levelOne or more correct optionsDifficulty 2 of 3ie087

A $0.1 M$ solution of a weak monobasic acid $HA$ ( $K_{a} = 1 0^{- 5}$ ) is diluted to $0.001 M$ at 298 K. Select the correct statement(s).

A $[H X^{+}]$ decreases.
BThe degree of ionisation becomes 100 times larger.
CThe degree of ionisation increases.
DThe pH rises by about 1 unit.

Answer: A, C, D

α = K_{a} / c

rises from

0.01

to about

0.095

(about 10 times, not 100 times).

[H X^{+}]

falls from

1 0^{- 3}

to about

9.5 \times 1 0^{- 5} M

, so the pH rises from 3 to about 4, i.e. by about 1 unit.

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