Gravimetric Analysis of Metal Carbonate
Introduction:
In this laboratory the identity of group 1 metal Carbonate is determined gravimetrically using a double
replacement precipitation reaction.
Concepts:


Double-Replacement reaction
Gravimetric analysis.
Background:
The identity of group 1 metal M is determined by analyzing an unknown Group 1 metal carbonate,
M2CO3.
There are 3 main reactions in this lab:
1. Equation 1: M2CO3 (s) → 2M+ (aq) + CO32- (aq)
2. Equation 2: Ca2+ (aq) + CO32-(aq) → CaCO3 (s)
3. Equation 3: CaCl2 (aq) + M2CO3→ CaCO3 (s) + 2MCl (aq)
The precipitated calcium carbonate is then filtered, dried, and weighed. The moles of calcium
carbonate, CaCO3, are equal to the moles of Group 1 metal carbonate, M2CO3, added to the original
solution. Dividing the mass of the unknown carbonate by the moles of calcium carbonate yields the
formula weight, and thus the identity, of the Group 1 metal carbonate.
Experiment Overview
The purpose of this lab is to determine the identity of a Group 1 metal carbonate compound by
gravimetric analysis. The unknown is weighed and dissolved in water. A solution of calcium chloride
is added to the metal carbonate solution to precipitate the carbonate ions as calcium carbonate. The
precipitate is filtered, dried, and weighed. From the data, the formula weight and identity of the
unknown metal carbonate is determined.
Pre-Lab Questions
1. From the mass of CaCO3, calculate the moles of CaCO3 precipitated.
2. Calculate the molar mass of the unknown (since CO3-2 came from M2CO3 we can assume 1
mole of CaCO3 = 1 mole of M2CO3)
3. Calculate the molar mass of the following Group 1 metal carbonates:
a. Li2CO3
b. Na2CO3
c. K2CO3
4. What is the identity of M2CO3?
5. Calculate the percent error in the molar mass determination of M2CO3 by comparing the
experimentally determined molar mass of M2CO3 to the known molar mass of the appropriate
metal carbonate.
**There is another way to solve Gravimetric Analysis problems, look in the Princeton Review book pg160.**
Mass of crucible + M2CO3
12.627 g
Mass of crucible
10.655 g
Mass of M2CO3
1.972 g
Mass of filter paper + CaCO3
2.436 g
Mass of the filter paper
0.598 g
Mass of the CaCO3
1.838 g
Moles of CaCO3
Molar mass of M2CO3
Mol
g/mol
Identity of M2CO3
Percent error
%
Materials:
Calcium Chloride solution, CaCl2, 0.2 M, 125 mL
Unknown sample M2CO3, 2g
Water, distilled or deionized, 200 mL
Scale
Beakers, 300-mL, 2
Hot Plate
Crucible, 15-mL
Crucible tongs
Filtered funnel
Filtered paper, quantitative
Stirring rods, glass, 2
Graduate Cylinders, 100mL, 250mL
Procedure
1.
2.
3.
4.
Obtain a clean, dry 15-mL crucible
Place the crucible on the hot plate (setting of 6-8) for 2-3 minutes with the cover off.
Using tongs remove the crucible and immediately cover. Record the mass in the Data Table
While the crucible is still on the balance, add approximately 2.00g of the unknown carbonate to
the crucible. Record the combined mass of the crucible and unknown carbonate in the Data
Table.
5. Place the crucible (with the cover off) on the hot plate for 5 minutes.
6. Let the crucible cool (with the cover on) and mass the crucible on an analytical balance. Record
the mass in the Data Table.
7. 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 masses.
8. Add the crucible contents to a 400-mL beaker
9. Add about 200 mL of distilled or deionized water to the beaker and stir to dissolve the unknown
carbonate.
10. Add about 125 mL of the 0.2 M CaCl2 solution to the 400-mL beaker and stir for 1 minute.
11. Let the precipitate settle (around 5 minutes).
12. Obtain a piece of quantitative filter paper. Mass the filtered paper on the analytical balance.
Record the mass of the filter paper in the Data Table.
13. Fold the filter paper to fit within the funnel (use distilled water to hold its shape if necessary).
14. Analyze your solution, you should notice a clear layer on top. Very carefully decant (the clear
liquid only) into a separate beaker or use a transfer pipette. This process maybe completed
multiple times throughout the filtration process.
15. Pour the solution into the filter funnel in small increments. Be sure to keep the liquid level
below the top of the filter paper.
16. Rinse the beaker with small amounts of distilled or deionized water from the wash bottle and
then transfers the washings to the filter.
17. When all of the solution has been transferred to the filter paper, rinse the solid with three small
portions of distilled or deionized water. Allow the funnel to drain completely.
18. Carefully remove the filter paper and place on a weighing dish to dry overnight. Place your
sample over by the windows.
19. The next day, weigh the filter paper with the solid CaCO3 on an analytical balance. Record the
mass in the Data Table.
Data Table
Mass of crucible + M2CO3
Mass of crucible + M2CO3 dried, 1st weighing
Mass of crucible + M2CO3 dried, 2nd weighing
Mass of Crucible
Mass of M2CO3
Mass of filter paper + CaCO3
Mass of Filter paper
Mass of CaCO3
Moles of CaCO3
Molar Mass of M2CO3
Identity of M2CO3
Percent Error
Unit
g
g
g
g
g
g
g
g
mol
g/mol
%
Calculations and Post Lab Analyses
1. Calculate the moles of precipitated calcium carbonate, CaCO3. Enter this value in the Data
Table. (show your work)
2. Calculate the molar mass of the unknown carbonate. Enter this value in the Data Table.
(show your work)
3. Identify the unknown and calculate the percent error in the molar mass value. Enter both
values in the Data Table. (show your work).
4. Review the procedure and list possible sources of error that would cause the molar mass of
the unknown to be (a) too high or (b) too low.