We did a lab to separate iron from aluminum ore. Here is some specifics on it th

Question

We did a lab to separate iron from aluminum ore. Here is some specifics on it that will help answer my question which is at the end... Background: In this experiment you will be given a mixture of Al(OH)3 and Fe2O3. You're job will be to separate out most of the iron by solution in KOH and then to remove the rest by crystallization of alum, which is KAl(SO4)2 • 12H2O. Procedure: weigh out on a platform balance 4.0g of the Al(OH)3 - Fe2O3 mixture (75% Al(OH)3 by weight). Place in a 200 mL beaker, and add 15 mL of water followed by 25 mL of 2 M KOH. Heat nearly to boiling while stirring constantly, keeping the mixture hot for 5 minutes so that the Al(OH)3 dissolves. Then filter and add 30 mL of sulfuric acid plus 3mL excess to supress hydrolysis of Al3+ to the filtrate in a small beaker. QUESTION: At the end of a week's time no crystals formed. What are some reasons this happened?

Explanation / Answer

According to the experimental procedure, the final volume of mixture in the 200 ml beaker is:

V = 15ml (H2O) + 25ml (KOH) + 30 ml (H2SO4)* + 3 ml (excess)*
V = 73 ml
*did not specified acid concentration

Now, assuming the procedure is done perfectly, you should obtained the next quantity of alum:

In 4.0 g of ore, 75% is Al(OH)3:
m Al(OH)3 = 4.0 g . 75/100
m Al(OH)3 = 3.0 g
In mol:
n Al(OH)3 = 3.0 g/78.0036 g/mol
n Al(OH)3 = 0.03846 mol

Assuming 100 % conversion to alum (KAl(SO4)2 . 12 H2O):
m.alum = 0.03846 mol . 474.3884 g/mol
m.alum = 18.245 g

This compound solubility in water is 14.00 g/100 ml at 20 ºC.
Since the theoretical mass of alum clearly exceed the solubility, you should have a precipitate at the end of a week's time.

If no crystals formed, one error could have been a bad separation of iron as Fe(OH)3 from ore.
In alkaline solution both metals precipitate as Al(OH)3 and Fe(OH)3.
At high temperatures, the solubility in water of Al(OH)3 is increased while Fe(OH)3 not that much.
If the heat treatment is not appropriate, Al(OH)3 and Fe(OH)3 are filtered, and without aluminum (or not enough) in solution you cannot produce alum.

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