INORGANIC cHEMISTRY -II

1. Materials required

2. Unknown solid sulfate salt, 6 M HCl solution, and 0.1 M BaCl2 solution, 250-mL beaker, analytical balance, stirring rod, 100-mL graduated cylinder, stand with ring clamp, wire screen, Bunsen burner, wash bottle with distilled water, crucible and lid, crucible tongs, ash-less filter paper, large funnel, 500-mL Erlenmeyer flask, clay triangle

Procedure

1. Weigh a clean, dry 250-mL beaker to the nearest 0.001 g using the analytical balance, and record this mass on your lab report. Next, add 0.30 – 0.35 grams of your unknown sample to the beaker. Record the combined mass of the beaker plus sample on your lab report.

2. Add 50 mL of distilled water, followed by 20 drops of 6 M HCl (aq), to the sample in the beaker. Stir the contents of the beaker until the sample has entirely dissolved. Leave the stirring rod in the beaker.

3. Obtain a stand with a ring clamp from the back of the lab. Place your wire screen on the ring and the beaker containing your dissolved sample on the wire screen. Use the Bunsen burner to heat the solution until it is nearly (but not quite) boiling. Turn the Bunsen burner off before the solution boils.

4. While heating the solution, measure out 25 mL of 0.1 M BaCl₂ (aq) using a 100-mL graduated cylinder. The graduated cylinder used should be clean (rinse with distilled water) but does not need to be dry.

5. Slowly add small portions of BaCl₂ (aq) to the beaker containing the hot solution. You should observe the formation of a white precipitate of BaSO₄ (s). Stir the contents of the beaker as you add the BaCl₂ solution. The addition of the BaCl₂ must be performed very slowly – this step should take you at least 3 minutes to complete! When finished, rinse any precipitate that remains on the stirring rod into the solution with a small amount of distilled water, and then allow the precipitate to settle in the beaker for about 20 minutes.

6. While the precipitate settles, prepare your crucible by heating it in the hottest part of the Bunsen burner flame for about 2 minutes (use the crucible tongs, cradling the crucible as demonstrated by the instructor – never pick up a crucible by pinching the walls). Repeat with the lid. Place the hot crucible and lid on the metal base of the stand to cool. Once they have cooled to room temperature weigh the crucible without the lid using the analytical balance, and record this mass on your lab report. You do not need to weigh the lid.

7. Obtain a piece of ash-less filter paper from your instructor and fold it into quarters. Open the folded paper into a cone and place it into your large funnel. Wet the filter paper with a small amount of distilled water so that it adheres to the funnel. Sit the funnel in the mouth of a 500-mL Erlenmeyer flask, which will be used to collect the filtrate.

8. After 20 minutes has passed, slowly pour the mixture containing the BaSO4 precipitate down your stirring rod into the funnel. Be careful that the level of liquid in the funnel is never more than three-fourths of the way to the top of the filter paper. When the transfer is complete use your wash bottle (filled with distilled water) to rinse the residual precipitate from the beaker and the stirring rod into the funnel.

9. After all the liquid has drained from the funnel, wearing gloves very carefully press the top edges of the filter paper together, and gently fold the filter paper into a compact package that will fit into the crucible. It is important that you do not use too much force in order to avoid tearing the filter paper. Place the folded filter paper into the crucible.

10. Take your stand, ring clamp and Bunsen burner over to a fume hood. Place your clay triangle on the ring and the crucible in the clay triangle for support. Gently heat the crucible without the lid to remove the water. Once the paper appears to be dry (after several minutes), heat the crucible more vigorously so that the filter paper begins to char (turning from white, to brown, to black) – but not so vigorously that the filter paper bursts into flame. If the filter paper bursts into flame you should cover it with the crucible lid to put out the flame, then reduce the amount of heat and remove the lid. Continue to heat moderately with the lid off until all of the filter paper has turned black.

11. Once all the filter paper has turned black, vigorously heat the crucible without the lid in the hottest part of the Bunsen burner flame so that the bottom of the crucible is red hot. The center of the flame should now be directly on the crucible. The charred filter paper (carbon) will gradually combust and be converted into CO2 gas. When the filter paper is entirely combusted only the white BaSO4 should remain in the crucible. Continue to heat the crucible vigorously until no charred filter paper remains. This should take 5 - 10 minutes.

12. Allow the crucible to cool to room temperature (this takes at least 5 minutes). Weigh the crucible without the lid and its contents on the analytical balance. Record this mass.

13. Place the crucible and its contents back in the clay triangle and heat vigorously with the lid off for an additional 5 minutes. Then allow it to cool again and reweigh the crucible and its contents without the lid. If the mass is within 0.005 grams of that in step 12, then record this mass on your lab report. If the mass has decreased by more that 0.005 grams, then either the BaSO4 is still wet or not all of the filter paper has combusted and you should repeat this step until you achieve a consistent mass, making sure the crucible is in the hottest part of the flame.

14. Discard the BaSO4 in the proper waste container, then clean up as directed by your instructor.