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Part 1: preparation of alum/quantative analysis (calculation of % yield)

1.     Cut a 5cm x 5cm square piece of aluminum from a scrap aluminum can and sand the paint off using steel wool. 

2.     Cut this piece into smaller pieces (about 0.5 cm long) and weigh out ~1g. Record the weight to 3 decimal places.

3.     Place the aluminum in a 400-mL beaker and add 50 mL of 4 M KOH.

4.     Place beaker on the heating mantle and heat gently.


5.     When the bubbles have stopped, remove from the heat, i.e., hydrogen is no longer evolved.

6.     Vacuum filter the solution and save the filtrate, you may toss out the solid. (Ask your TA if you need instruction in performing a vacuum filtration.)

7.     Carefully rinse your graduated cylinder, pour out 25-30 mL of 9 M H 2 SO 4 size 12{H rSub { size 8{2} } ital "SO" rSub { size 8{4} } } {} and then add slowly to the filtrate.

CAUTION - H 2 SO 4 size 12{H rSub { size 8{2} } ital "SO" rSub { size 8{4} } } {} IS A STRONG ACID AND DEHYDRATOR. SEE TA IMMEDIATELY IF YOU SPILL ANY!

8.     Heat gently while stirring until the solution becomes clear. Boil the solution down to a volume of about 45 mL.

7.     While the solution is boiling, prepare an ice bath by filling a bowl half-way with ice and then adding water until the bowl is three-quarters full.

8.     After the "boiling off", let the beaker cool to room temperature and then place it in the ice bath. Crystals of alum should form. Allow to cool for 15 minutes.

9.     After 15 minutes, vacuum filter the product and wash with 10 mL of ethanol.

10. Allow to dry for a few minutes. Dry completely in the microwave at less than 50% power in 10 sec intervals until the weight no longer changes.

11. Calculate the percent yield and describe the appearance of the crystals.

Part 2: qualitative analysis of alum

1.     Use a straw spatula to transfer a few of the alum crystals (about 5 mg) to a watch glass. Add 3 drops of water to the crystals. Stir gently until the crystals dissolve.

2.     Use a small piece of indicator paper to see whether the solution is acidic, basic, or neutral.

3.     Now add 1 drop of 0.5 M BaCl 2 size 12{ ital "BaCl" rSub { size 8{2} } } {} (barium chloride) to the solution. Record your observations.

4.     A really good test for the presence of potassium is a flame test. Using the hot grips, hold a stainless spatula in the flame of a Bunsen burner to volatilize impurities from the spatula.

5.     When one end of the spatula is red hot, remove it, and quickly touch it to a small cluster of crystals. Several should stick.

6.     Slowly bring the spatula (plus crystals) toward the flame and watch carefully. Hold the crystals in the flame for at least 5 seconds (until the solid glows). Record your observations.

7.     Remove the spatula and place on a non-asbestos mat.

Part 3: acid-base properties of aluminum compounds

Clean 3 test-tubes and label them.

Tube 1:

Place 10 drops of 1M aluminum nitrate solution and 2 drops of 6.0 M sodium hydroxide solution, mix well and record your observations on your lab report Then add more sodium hydroxide solution to tube 1 until a reaction is observed (around 7 drops). Mix well. Record your observations.

Tube 2

Place 10 drops of 1M aluminum nitrate solution and 2 drops of 6.0 M sodium hydroxide solution, mix well and record your observations on your lab report Then add 4 mL of 6M HCl and record your observations.

Tube 3

Add 3 mL of 6M ammonium hydroxide, i.e aqueous ammonia to your alum and record your observations.

Questions & Answers

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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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Source:  OpenStax, General chemistry lab spring. OpenStax CNX. Apr 03, 2009 Download for free at http://cnx.org/content/col10506/1.56
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