▪ Objectives

▪ Introduction

▪ Experimental Procedure &

Result Videos

▪ Report Sheet

▪ Evaluation Criteria

OBJECTIVES

• To prepare an alum from an aluminum can or foil

• To test the purity of the alum using a meting-point measurement

IntroductionAn alum is a hydrated double sulfate salt with the general

formula:

X+M3+(SO4)2‧12H2O

Common household alum is ammonium aluminum sulfate

dodecahydrate:

NH4+Al3+(SO4)2‧12H2O

In this experiment, you will synthesize potassium aluminum sulfate dodecahydrate from an aluminum foil.

Aluminum reacts with KOH to produce potassium aluminate salt solution and hydrogen gas:

2 Al(s) + 2 K+(aq) + 2 OH-

(aq) + 6 H2O(l) → 2 K+

(aq) + 2 Al(OH)4-(aq) + 3 H2(g)

Then, the aluminate ion is treated with sulfuric acid to form a precipitant, aluminum hydroxide.

2 K+(aq) + 2 Al(OH)4

-(aq) + 2 H+

(aq) + SO42-

(aq) →2 Al(OH)3(s) + 2 K+

(aq) + SO42-

(aq) + 2 H2O(l)

Aluminate hydroxide redissolves after heating.

2 Al(OH)3(s) + 6 H+(aq) + 3 SO4

2-(aq) ՜

∆

2 Al3+(aq) + 3 SO4

2-(aq) + 6 H2O(l)

Finally, potassium aluminum sulfate dodecahydrate forms octahedral-shaped crystals when cooled.

K+(aq) + Al3+

(aq) + 2 SO42-

(aq) + 12 H2O(l)→

KAl(SO4)2‧12H2O(s)

EXPERIMENTAL

PROCEDURE & RESULT

VIDEOS

Overview

A known mass of starting material is used to synthesize thepotassium alum. The synthesis requires the careful transfersolutions and some evaporation and cooling techniques.

Major worksEstimated Time En route (min)

Reaction of aluminum foil with 4 M KOH and Filtration 10 ̴ 15

Addition of 6 M H2SO4 ̴5

Heating for Dissolution of Al(OH)3 with stirring 20̴30

Crystallization 40

Suction Filtration and Drying 5̴10

Drying in a oven 20

Melting point determination 20̴30

A. Potassium Alum Synthesis

1. Prepare the aluminum sample

1) Obtain a piece of aluminum foil and weigh out about 1.0 g(±0.01 g) of aluminum pieces.

2) Tear the foil into small pieces and place the pieces in a 250 mLbeaker.

Mass of aluminum foil: 1.037g

2. Dissolve the aluminum pieces

1) Move the beaker to a fume hood. Carefully add 50 mL of 4 MKOH to the aluminum pieces, and swirl the reaction mixture.Notice that the reaction is exothermic.

2) Allow the reaction to proceed until all of the foil is dissolved.

3. Filter the reaction mixture

1) When no further reaction is evident, return the reaction mixtureto the laboratory desk.

2) Carefully pour the reaction mixture through your Bűchner funneland filter flask setup, and rinse the filter paper with a smallamount of distilled water.

Filtration set up

Vacuum source

Rubber tube

Bűchnerfunnel

Vacuum flask

4. Allow the formation of aluminum hydroxide

1) Add a stir bar into the beaker. With constant stirring on astirrer/hot plate, slowly add 30 mL of 6 M H2SO4 to the coolsolution. Solid aluminum hydroxide may form.

5. Dissolve the aluminum hydroxide

GENTLY heat on a hot plate for 20 ̴ 30 min the reaction mixturewith stirring until all of the aluminum hydroxide dissolves. At the endof the heating period the volume of the solution should be about 60ml.

1) If there are no solid impurities present, proceed to the next step.If solid impurities are present, pour the mixture through the clearBűchner funnel and filter flask to separate undissolved solid fromthe mixture.

6. Crystallize the alum

1) Remove the solution from the heat.

2) Cool the solution in an ice bath for 40 minutes. Alum crystals shouldform within 20 minutes. If crystals do not form, use a hot plate(Figure 15.2) to gently reduce the volume by one third to one-half(do not boil!) again and return to the ice bath.

7. Isolate and wash the alum crystals

1) Using Bűchner funnel, isolate the alum crystals form the

solution.

2) Wash the crystals on the filter paper with two (cooled-to-ice

temperature) 10-mL portions of a 50% (by volume) ethanol-

water solution.

3) Maintain the vacuum suction for at least 5 minutes until the

crystals appear dry. And place a aluminum dish with the

crystals into a pre-heated oven (̴70℃) for 20 minutes.

4) Weigh and record the mass (0.01g) of the crystals.

8. Percent yield

Calculate the percent yield of your alum crystals.

Product: 29.421g

Clay triangle

Bunsen burner

CLEANUPRinse all glassware twice with tap water and twice with distilled water. All rinses can be

discarded in the waste container, followed by a generous amount of tap water.

DISPOSALDiscard the filtrate in the container labeled

“Student Prep Alum”.

B. Melting Point of the Alum

1. Prepare the alum in the melting-point tube.

1) Place finely ground, dry alum to a depth of about 0.5 cm in thebottom of a melting point capillary,

2) To do this, place some alum on a piece of dry filter paper and“tap-tap” the open end of the capillary tube into the alumuntil the alum is at a depth of about 0.5 cm (Figure 15.3, page196).

3) Invert the capillary tube and compact the alum at the bottomof the tube (see Figure 15.4).

2. Determine the melting point of the alum

Use the apparatus in Figure 15.6.1) Place the capillary tube containing the sample into the melting-point

apparatus.2) Mount the capillary tube containing the sample beside the digital

temperature probe (Figure 15.6 insert) with a rubber band. Transferthe sample/thermometer into the water bath.

3) Slowly heat the sample at about 3℃ per minute while carefullywatching the alum sample.

4) When the solid melts, note the temperature. Allow the sample to coolto just below this approximate melting point; at a 1℃ per minuteheating rate, heat again until it melts.

Melting point measurement set up

Lab quest

Temperature

probe

Capillary

Hot

plate

Water bath

5) Repeat the cooling/heating cycle until reproducibility isobtained-this is the melting point of your alum.

6) Record this on the Report Sheet.

▪ Temperature starts to melt around ~90.7 °C (1st

& 2nd measurements)

▪ Temperature completelymelted: 91.3 °C(1st)

▪

91.5°C (2nd)

Clay triangle

Bunsen burner

DISPOSAL

Dispose of the melting point tube in the Glass Only Waste box.

REPORT SHEET

A. Potassium Alum Synthesis

1. Mass of aluminum (g)

2. Mass of alum synthesized (g)

3. Theoretical yield (g)

4. Percent yield (g)

B. Average Point of the Alum

1. Melting point (℃)

2. Average melting point (℃)

The melting point of potassium alum is 92.5 ℃. Comment on the purity your sample

based on your experimental melting point.

Show calculation of theoretical yield.

EVALUATION CRITERIA

▪ Pre-lab assignment (20 points)- Summary: Introduction(Theory) and

Procedure (10 pts each)- Pre-laboratory question (20 pts)

▪ Lab Report (40 pts)▪ Post-lab assignment: Laboratory

Questions (20 pts)