Question

A precipitate of Fe(OH)3 is contaminated with Mg(OH)2. Explain how to can get rid of the contamination.

Answer 1

Iron(lll) is amphoteric so you could treat the mixture with alkali causing the aluminium hydroxide to dissolve:

Fe(OH)3(s) + OH−(aq) → Fe(OH)−4(aq)

Then filter the mixture to separate the solid magnesium hydroxide.

To recover the Iron(lll) hydroxide you treat the filtrate with mineral acid:

Fe(OH)−4(aq) + H+(aq) → Fe(OH)3(s) +H2O(l)

Answer 2

To get rid of the contamination of Mg(OH)2 in a precipitate of Fe(OH)3, you can utilize the difference in solubilities of the two compounds.

Mg(OH)2 is more soluble in water compared to Fe(OH)3. Therefore, one method to remove the contamination is by selectively dissolving Mg(OH)2 while leaving Fe(OH)3 behind. Here's a step-by-step procedure:

1. Obtain the contaminated precipitate of Fe(OH)3 mixed with Mg(OH)2.

2. Add water to the mixture to create a suspension. Stir the mixture to ensure thorough mixing.

3. Allow the suspension to settle. Since Mg(OH)2 is more soluble, it will dissolve in the water and remain in the solution, while Fe(OH)3 will settle at the bottom.

4. Carefully decant or separate the supernatant liquid (the water with dissolved Mg(OH)2) from the settled Fe(OH)3 precipitate. This can be done by pouring off the liquid without disturbing the settled precipitate.

5. Repeat steps 2-4 multiple times if needed to ensure maximum removal of Mg(OH)2 contamination.

6. Finally, collect the remaining Fe(OH)3 precipitate, which should now be free or significantly reduced in Mg(OH)2 contamination.

By selectively dissolving the more soluble Mg(OH)2 and separating it from the less soluble Fe(OH)3, you can effectively remove the contamination and obtain a purified precipitate of Fe(OH)3.