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 paper in half and crease. Next, fold the filter paper almost in half again, leaving about a 5° angle between the folded edges. 15. Tear off the corner of the top edge, open the filter paper into a cone shape and place the torn corner in the bottom of the cone. 16. Place the cone into the filter funnel. Position the paper tight against the funnel walls and moisten the paper with about 5 mL of distilled water from a wash bottle.*Note: After adding the water, use a finger to seal the paper tightly against the sides of the funnel so that little, if any air gaps are visible in the stem as the water filters through. 17. Set up the ring stand and iron ring and place the funnel in the ring. Let the funnel drain into a second 400-mL beaker. 18. Use a stirring rod; decant the liquid from 400-mL beaker into the funnel. Be sure the keep the liquid level below the top of the filter paper cone. 19. When all but approximately 10 mL of the liquid has been transferred, swirl the beaker to suspend the precipitated CaCO3. Transfer this to the funnel, again making sure not to fill the cone above the top of the filter paper. 20. Rinse the flask with small amounts of distilled water from the wash bottle and then transfer the washings to the filter. 21. When all the solid has been transferred to the filter paper, rinse the solid with three small portions of distilled water. Allow the funnel to drain completely. 22. Obtain a watch glass and take the filter paper out of the funnel and place it on the watch glass. Be careful not to tear the filter paper or lose the solid. 23. Let the solid in the filter paper dry.
24. When dried, weigh the filter paper and the solid CaCO3 on an analytical balance. Record the mass. 25. Repeat weighing until the mass readings do not change by .005g.
Data Table: Mass of Substance
Substance| Sample A (K2CO3)|
Mass of crucible| 11.48 g|
Mass of crucible + M2CO3| 13.48 g|
Mass of crucible + M2CO3 (dried) (1st weighing)| 13.79 g| Mass of crucible +M2CO3 (dried) (2nd weighing)| 13.37 g| Mass of M2CO3| 2.24 g|
Mass of filter paper | 1.13 g|
Mass of filter paper +CaCO3 (1st weighing)| 2.80 g|
Mass of filter paper + CaCO3 (2nd weighing)| 2.71 g|
Mass of filter paper +CaCO3 (3rd weighing)| 2.78 g|
Mass of CaCO3| 1.63 g|
Analysis Table: Identity of Unknown Carbonate
Substance| Sample A (K2CO3)|
Moles of CaCO3| .0163|
Molar mass of K2CO3| 137 g/mol|
Identity of M2CO3| K2CO3|
Percent Error| 1.5%|
CaCl2 (aq) + K2CO3 (aq) → CaCO3 (s) +2 KCl (aq)|
Potassium carbonate was identified by gravimetrically using a double-replacement precipitation reaction. The percent error was 6.6% and was most likely caused by a couple sources of error. The mass of the unknown carbonate increased since Group 1 carbonates are hydroscopic. The weight of the unknown carbonate increases due to the water absorbed from the air. The unknown carbonate in the filter paper had to be weighed a few times until it no longer changed. The data table shows that the unknown carbonate changed in weight after being weighed the first time and this step is very important because it is necessary that the carbonate samples are anhydrous when weighed. Another source of error would be the precipitate stuck in the beaker after the decanting process. The weight would not be accurate because some of the precipitate was stuck in the beaker and would not be accounted for when the calcium carbonate was weighed. The weight of the calcium carbonate fluxuated since carbonates are hydroscopic and the weight was averaged to help avoid a high percent error.