At what $pH$ must one work to the maximum to have a solution of iron(III) chloride where the iron(III) ion is hydroxylated at more than $6\%$?
$Fe(H_2O)_6^{3+}) $: weak acid
When the hydroxyl ion is hydroxylated at less than $6\%$, then its hydroxylated form will take more than $94\%$,
thus:
$\frac{[Fe(H_2O)_6^{3+}]}{[Fe(H_2O)_5OH^{2+}]}\gt \frac{94}{6}$
$\frac{[Fe(H_2O)_6^{3+}]}{[Fe(H_2O)_5OH^{2+}]}$ $=$ $\frac{[H_3O^+]}{K_a}$
thus:
$[H_3O^+]\gt \frac{94}{6}K_a$
$-log[H_3O^+]$ $\lt$ $-logK_a$ $-$ $log\frac{94}{6}$
$pH$ $\lt$ $ pK_a$ $-$ $log\frac{94}{6}$
$pH\lt 2.6$
