January 10th, 2012
Purpose: To gravimetrically identify a Group 1 metal carbonate using a double-replacement precipitation reaction.
1. Obtain a clean, dry 15-mL crucible.
2. Set up a Bunsen burner on a ring stand beneath a ring beneath a ring clamp holding a clay pipe stem triangle. Place the crucible in the clay triangle. Do NOT light the Bunsen burner. 3. Light the Bunsen burner and brush the bottom of the crucible with the burner flame for about one minute. Turn off the Bunsen burner and allow the crucible to cool. 4. Using tongs to handle the crucible, measure the mass of the clean, empty crucible to the nearest 0.001 g. Record the mass. 5. While the crucible is still on the balance, add approximately 2 g of the unknown carbonate to the crucible. Record the combined mass of the crucible and unknown carbonate. 6. Place the crucible on the clay triangle. Light the Bunsen burner again and slowly heat he crucible by brushing the bottom of the crucible with the Bunsen burner flame for 2-3 minutes. Set the crucible to cool on a heat-resistant pad. 7. Weigh the crucible on an analytical balance. Record the mass. 8. Repeat steps 6 and 7 until the mass of the crucible and unknown carbonate no longer decreases. *Note: The Group 1 metal carbonates are hydroscopic- they absorb water from the air. These heating steps are necessary to ensure the crucible is dry and the carbonate samples are anhydrous when massed. 9. Add the crucible contents to a 40-mL beaker.
10. Add about 200 mL of distilled water to the beaker and stir to dissolve the unknown carbonate. 11. Add about 125 mL of the .2 M CaCl2 solution to the 400-mL beaker and stir. 12. Let the precipitate settle for about 5 minutes.
13. Obtain a piece of quantitative filter paper. Weigh the filter paper on the analytical balance. Record the mass of the filter paper. 14. Fold the filter paper into a cone. First fold the filter...