A mixture contains both and ions

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

Problem

A mixture contains both Fe2+\text{Fe}^{2+} and Ni2+\text{Ni}^{2+} ions, each at a concentration of 0.010 mol L−10.010\,\text{mol L}^{-1}. You are asked to selectively precipitate Fe(OH)2\text{Fe(OH)}_2 while keeping Ni2+\text{Ni}^{2+} in solution at 25 ∘C25\,^\circ\text{C} by adjusting the pH. Solubility-product data at 25 ∘C25\,^\circ\text{C}: Fe(OH)2(s): Ksp=1.6×10−15\text{Fe(OH)}_2(s):\ K_{sp} = 1.6\times10^{-15} Ni(OH)2(s): Ksp=6.0×10−16\text{Ni(OH)}_2(s):\ K_{sp} = 6.0\times10^{-16} Assume activity coefficients are unity. (a) Determine the minimum hydroxide-ion concentration at which Fe(OH)2\text{Fe(OH)}_2 just begins to precipitate from the mixture. (b) At the hydroxide-ion concentration found in part (a), calculate the ratio [Ni2+]present[Ni2+]initial\frac{[\text{Ni}^{2+}]_{\text{present}}}{[\text{Ni}^{2+}]_{\text{initial}}}. (c) Compute the maximum pH at which at least 99.0% of the Fe2+\text{Fe}^{2+} originally present is precipitated while keeping less than 5.0% of Ni2+\text{Ni}^{2+} precipitated. (d) Briefly justify whether further raising the pH beyond the value in part (c) would compromise selectivity between the two metal ions.

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