Gravimetric Analysis of a Metal Carbonate
Introduction: Pre-lab Assignment
A large variety of techniques and procedures, ranging from instrumental methods such as spectroscopy and
chromatography to more classical processes, such as qualitative and gravimetric analyses, have been created to
accomplish the task of determining the identity of a compound. In this lab, the identity of a Group I metal carbonate
is determined gravimetrically using a double-replacement precipitation reaction.
Background:
In this experiment, an unknown Group I metal carbonate, M2CO3, is analyzed to determine the identity of the Group I
metal, M.
A known amount of the soluble unknown carbonate is dissolved in water to dissociate the compound into its ions
(Equation 1).
M2CO3 (s)  2 M+ (aq) + CO32- (aq)
Equation 1
When a solution of calcium chloride, CaCl2, is added to this metal carbonate solution, a precipitate of calcium
carbonate forms (Equation 2).
Ca2+ (aq) + CO32- (aq)  CaCO3 (s)
Equation 2
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 I
metal carbonate.
Purpose:
To identify the identity of a Group I metal carbonate compound by gravimetric analysis.
Procedure:
The Group I metal carbonate has been stored in a desiccation chamber and should be completely dry upon receipt.
1.
2.
3.
4.
5.
6.
7.
8.
Measure approximately 2 g of the Group I metal carbonate and accurately record the mass of the sample.
Place the metal carbonate sample in a 400 mL beaker.
Add about 200 mL of DI water and stir to completely dissolve the metal carbonate.
Allowing for the metal carbonate to be the limiting reactant in the experiment, calculate the amount of 0.2 M
CaCl2 solution you will add to the reaction.
Add the volume of 0.2 M CaCl2 determined in step 4 to the 400 mL beaker and stir.
Let the precipitate settle (at least 5 minutes).
Obtain a piece of quantitative filter paper, weigh and record the mass of the filter paper.
Prepare the filter and funnel.
a. First fold the filter paper in half and crease.
b. Next fold the filter paper almost in half again, leaving about a 5o angle between the folded edges.
c. Tear off the corner of the edge, open the filter paper into a cone shape and place the torn corner in the
bottom of the cone.
d. Position the paper tight against the funnel walls and moisten the paper with about 5 mL of DI water.
Note: After adding the water, use index fingers to seat the filter 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.
9. Set up a ring stand and iron ring and place the funnel in the ring. Let the
funnel drain into a second 400 mL beaker.
10. Using a stirring rod, decant the liquid from the 400 mL beaker into the
funnel. Be sure to keep the liquid level below the top of the filter paper
cone.
11. 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.
12. Rinse the flask with small amounts of DI water and transfer the washings to
the funnel.
13. Allow the funnel to drain completely.
14. Obtain a watch glass. Using a spatula, take the filter paper out of the funnel
and place it in the center of the watch glass. Be careful not to tear the paper
or lose any part of the solid.
15. Using the spatula, carefully open the filter paper into a circle on the watch
glass.
16. Place the watch glass in the hood and allow the filter paper to dry until the
next class time.
17. When dry, weigh and record the filter paper and the solid CaCO3.
Calculations:
1.
2.
3.
4.
Calculate the moles of precipitated calcium carbonate, CaCO3.
Calculate the molar mass of the unknown carbonate.
From the calculated molar mass, identify the unknown.
Calculate the percent error in the molar mass value.
Discussion Questions:
1.
2.
3.
4.
5.
6.
The alkali metal carbonate provided as the unknown was stored in a dessicator to ensure that it was dry (no
water bound to the compound) before you began the experiment.
a. Why was it necessary to make sure there was no water present in the sample?
b. If this precaution hadn’t been taken what might you have done to ensure a sample without water?
Why is calcium chloride solution used in the double replacement reaction instead of potassium chloride?
What volume of CaCl2 did you decide to use in step 4 of the procedure? Explain how you arrived at this
volume.
Although many of you proposed using vacuum filtration, the provided procedure recommends the use of
gravity filtration. Why might gravity filtration be a better choice than vacuum filtration?
Based on your calculations, what is the identity of the unknown alkali metal carbonate?
What are some potential sources of error in this experiment. Explain.