A solution initially contains and

CHEMISTRY · Single correct · AP · Chemistry · Equilibrium · Equilibrium

Problem

A solution initially contains

0.010\,\mathrm{M}\,\mathrm{Fe^{3+}}

and

0.010\,\mathrm{M}\,\mathrm{Cu^{2+}}

. As the pH is raised slowly from 1 to 9, hydroxide salts may precipitate. Given

K_{sp}(\mathrm{Fe(OH)_3}) = 4 \times 10^{-38}

and

K_{sp}(\mathrm{Cu(OH)_2}) = 2 \times 10^{-20}

, which statement best predicts the order of precipitation? (a)

\mathrm{Cu(OH)_2}

precipitates before

\mathrm{Fe(OH)_3}

because its larger

K_{sp}

indicates lower solubility. (b)

\mathrm{Cu(OH)_2}

precipitates before

\mathrm{Fe(OH)_3}

because Cu

^{2+}

needs only two

\mathrm{OH^-}

ions, whereas Fe

^{3+}

needs three. (c)

\mathrm{Fe(OH)_3}

precipitates before

\mathrm{Cu(OH)_2}

because it reaches its

K_{sp}

at a lower

\mathrm{[OH^-]}

. (d) Both hydroxides begin to precipitate at the same pH since the metal-ion concentrations are equal.