The. Analysis of Alum
After a compound has been synthesized, tests should be carried out to verify that the compound fonned is indeed the compound desired. There are a number of tests that can be perfonned to verify that the compound is the one desired. In Experiment 15A, you prepared alum crystals, KAl(S04)z·12H 20. In Experiment 15B, you will conduct a series of tests to detennine if your crystals are really alum. The first test is to find the melting temperature of the compound and compare this value with the accepted (published) value for alum (92.5°C). The second test detennines the water of hydration present in the alum crystals. The third test is a chemical test to detennine the percent sulfate in your sample of alum.
In this experiment, you will • • • • Detennine the melting temperature of a sample of alum. Detennine the water of hydration of a sample of alum. Detennine the percent sulfate of a sample of alum. Verify the chemical fonnula of a sample of alum.
Part I, Melting Temperature Test Materials
Vernier computer interface computer Temperature Probe capillary tubes 150 mL beaker or Thiele melting-point tube mortar and pestle glass stirring rod Part II, Water of Hydration Test Materials
alum crystals, KAl(S04)2·12H20 hot plate ring stand, ring, and wire gauze utility clamp cork or split stopper small rubber band
crucible with cover balance lab burner
Patt III, Percent Sulfate Test Materials
alum crystals, KAl(S04)z·12H20 ring stand, ring, and clay triangle tongs or forceps alum crystals, KAl(S04)z·12H20 0.20 M barium nitrate solution ring stand, ring, and wire gauze 50 mL or 100 mL graduated cylinders glass stirring rod
250 mL beaker watch glass to fit the 250 mL beaker balance Gooch crucible filter apparatus rubber policeman or other scraping tool
Advanced Chemistry with Vernier
Part I Determine the Melting Temperature of Alum
1. Obtain and wear goggles. 2. Connect a Temperature Probe to Channell of the Vernier computer interface. Connect the interface to the computer with the proper cable. 3. Start the Logger Pro program on your computer. Open the file "15b Alum" from the Advanced Chemistry with Vernier folder. 4. Use a mortar and pestle to pulverize about 0.5 g of dry alum and place it in a small pile in the mortar. Push the open end of a capillary tube into the pile of the alum powder. Pack alum into the capillary tube to a depth of about 1 cm by tapping the tube lightly on the table top. 5. Use a rubber band to fasten the capillary tube to the Temperature Probe. The tip of the tube should be even with the tip of the probe. Use a utility clamp to conriect the Temperature Probe to a ring stand. If necessary, place the probe in a split stopper or a cork to secure it in the clamp (see Figure 1).
6. Prepare a water bath to be heated by a hot plate. Your instructor may also direct you to use a Thiele tube. If you do not use a Thiele tube, stir the water bath throughout the testing to maintain a constant bath temperature. 7. Click ~t] to begin data collection. Immerse the capillary tube and Temperature Probe in the water bath. Warm the alum sample at a gradual rate so that you can precisely determine the melting temperature. The white powder will become clear when it is melting. Observe the temperature readings and record the precise melting temperature when the substance is completely clear. 8. Conduct a second test with a new sample of alum in a new capillary tube.
15B - 2
Advanced Chemistry with Vernier
The Analysis ofAlum
Part II Determine the Water of Hydration of Alum
9. Heat a crucible with cover over a burner flame until it is red hot. Allow the crucible to cool, and then measure the total mass of the crucible and cover. Handle the crucible with tongs or forceps to avoid getting fingerprints on it. 10. Place about 2 g of your...