GRAVIMETRIC ANALYSIS OF AN UNKNOWN METAL CARBONATE
Background:
Gravimetric analysis is a standard classical quantitative method where a precipitate is isolated, purified, dried, and
massed. In this experiment, an unknown alkali metal carbonate, M2CO3, is analyzed to determine the identity of the
metal. The following reaction is performed: CaCl2 (aq) + M2CO3 (aq)  CaCO3 (s) + 2MCI(aq)
The precipitated calcium carbonate is then filtered, rinsed, dried, and massed. The molar mass of the alkali metal
carbonate can be determined using stoichiometry. Since the metal carbonate will be the limiting reactant, the number
of moles of CaCO3 formed can determine the number of moles of the unknown carbonate and the molar mass can be
calculated.
Purpose:
To determine the identity of an alkali metal carbonate using gravimetric analysis of a precipitation reaction.
Materials:
Unknown metal carbonate
Calcium chloride solution, 0.25 M
Distilled water
Buchner funnel
Filter Paper
Side-arm Flask
Electronic Balance
Beaker, 250 mL
Drying Oven
Watch glass
Stirring rod
Graduated Cylinder, 100 mL
Pre-lab questions:
1. Why is a calcium chloride solution used instead of a solution of potassium chloride?
2. What is the excess reactant in the reaction?
3. What is the purpose of rinsing the precipitate (on the filter paper) with distilled water?
4. What is the purpose of breaking up your solid precipitate and putting it in the drying oven a second time?
Procedure:
1. Mass out approximately 2 g of unknown carbonate.
2. Add your massed unknown carbonate to a 250-mL beaker. Add about 100 mL of distilled water and stir.
3. Add about 125 mL of 0.25 M CaCl2 and stir. Let the precipitate settle (2 min).
4. To verify precipitation is complete, add an additional 25 mL of CaCl2.
5. Mass a piece of filter paper.
6. Insert the filter paper into the Buchner funnel. Wet the filter paper with distilled water.
7. Pour the contents of your beaker slowly into the funnel. Be careful as you pour so that
none of the mixture flows out of the filter paper or the funnel. Use a wash bottle of
distilled water to rinse the precipitate out of the beaker with small quantities of water.
Use a little more distilled water to wash the precipitate that now collected on filter
paper. Rinsing the precipitate will remove any spectator ions that are left on the filter
paper with the precipitate.
8. Place the filter paper on a watch glass and place in a drying oven for 10-15 minutes
9. Remove and find the mass of the sample.
10. Break up the solid and return to the drying oven for 5 minutes
11. Find the mass of the sample.
12. Dispose of precipitate and filter paper in trash can, empty side-arm flask, rinse and dry 250-mL beaker (make sure
no ppt remains in beaker)
Data:
Create your own data table from procedure above. Be descriptive and include units. Make sure to include your
unknown number.
Calculations:
Calculate the molar mass of your unknown. (Show all your work for all steps – clearly and neatly. Show all units and use
correct amount of significant figures.)
Post- lab questions:
1. Identify your unknown alkali metal carbonate. Justify your answer.
2. Calculate the percent error in the molar mass calculation by comparing the experimentally determined molar mass
of M2CO3 to the known molar mass of the appropriate alkali metal carbonate.
3. Would the mass of precipitate that you measured be larger or smaller if you did not wash the precipitate with
distilled water before drying it? Explain
4. A student spilled some of the unknown metal carbonate after massing it but before mixing it with calcium chloride.
How would the calculated value of the molar mass be affected – too high, too low, or remain the same?
Justify your answer.
5. If the precipitate was still not completely dry when massing it the second time, how would the calculated value of
the molar mass be affected – too high, too low, or remain the same? Justify your answer.