A weak acid solution is diluted ten times at constant temperature (alpha ll 1 before and after). Its [H+] becomes

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A weak acid solution is diluted ten times at constant temperature ( $α ≪ 1$ before and after). Its $[H X^{+}]$ becomes

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

, so

[H X^{+}] \propto c

. For

c \to c /10

the factor is

1/ 10 = 0.316

. A factor of

0.1

would hold only for a strong acid.