Iron forms two common hydroxide precipitates whose solubility controls the…

CHEMISTRY · Multi-part · AP · Chemistry · Equilibrium · Equilibrium

Problem

Iron forms two common hydroxide precipitates whose solubility controls the concentration of iron in anaerobic natural waters. The

K_{sp}

values at 25 °C are given.

Solid	Formula	$K_{sp}$
Iron(II) hydroxide	Fe(OH) $_2$	$4.9\times10^{-17}$
Iron(III) hydroxide	Fe(OH) $_3$	$2.8\times10^{-39}$
Iron(III) oxide-hydroxide	FeO(OH) (treat as FeO $_{1}$ OH)	$1.6\times10^{-44}$

Assume that only the dissolution equilibria for these solids establish the maximum dissolved iron concentration and that no oxidation–reduction processes occur. (a) For each solid, derive an algebraic expression that relates molar solubility

s

K_{sp}

. (b) Calculate

s

(in mol L

^{-1}

) for each iron compound. (c) Compute the ratio of the molar solubility of Fe(OH)

_2

to that of Fe(OH)

_3

. (d) Using your results, discuss how the oxidation state of iron influences its mobility in water and relate this trend to the values of

K_{sp}

Related theory

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