Lab3.2 Gravimetric Analysis of a Carbonate
Pre-lab HW: Found on your Ch1 – 3 HW sheet
Have you ever wondered how ores of precious metals like gold and silver are evaluated for their worth? This
evaluation is called an assay. One type of assay begins by measuring the mass of dry ore and then dissolving it. A salt
compound is added to the solution to form an insoluble precipitate with the desired metal. The precipitate is dried and
its mass is determined. Then, the percent composition by mass of the metal in the ore is calculated.
A similar analytical method is used to identify an unknown compound. You will use gravimetric analysis by mass to
determine the identity of an unknown Group I carbonate compound, X2CO3.
First, you will dissolve your unknown soluble carbonate compound in water:
X2CO3(s)  2X+(aq) + CO32-(aq)
2+
Next, you will precipitate all of the carbonate ions by using an excess Ca ions from aqueous calcium chloride.
Ca2+(aq) + CO32-(aq)  CaCO3(s)
The overall reaction for this precipitation reaction is:
CaCl2(aq) + X2CO3(aq)  CaCO3(s) + 2XCl(aq)
Then, you will filter and dry the CaCO3 precipitate and determine its mass.
𝑔𝑟𝑎𝑚𝑠
𝑚𝑜𝑙
Next, calculate the number of moles of CaCO3 by its formula mass conversion:
𝑥
= moles
1
𝑔𝑟𝑎𝑚𝑠
Since the molar ratios of CaCO3 and X2CO3 are 1:1, you can determine the formula mass for the X2CO3
by rearranging the equation above to solve for formula mass.
Finally, you will identity the Group I metal by subtracting the mass of carbonate (CO32-) from the experimental
formula mass of X2CO3.
Your teacher will assign your group one of two substances, either Unknown A or Unknown B. At the end of the
procedure, you will exchange data with a group that worked with the other unknown. Then, you will perform the
calculations and determine the identity of both unknowns.
Procedure
1. Determine the mass of a clean, dry crucible. Record the mass in the Data Table.
2. Add 1 scoop of your unknown to the crucible, re-mass, and record the
mass in the Data Table.
***Step 4 has already been done for you, so you can skip steps 3 – 6.***
3. Place the crucible at a 45˚ angle in a clay triangle, as shown.
4. Heat the crucible gently with back-and-forth movement of the flame
across the bottom of the crucible for 3 minutes.
This will remove any absorbed water from the unknown.
5. Remove the crucible, place it on the ring stand base, and allow to cool for
2 minutes. Caution: Prevent burns! Use tongs to transport the crucible.
6. Mass of the crucible and unknown again and record it in the Data Table.
7. Determine the mass of the unknown carbonate, record in the Data Table.
8. Place the contents of the crucible in a 250ml beaker. Add 50ml of distilled
water to the beaker, and then stir the contents with a stirring rod.
9. Rinse the crucible with a washer bottle, and add the washings to the beaker.
10. Add 25ml of 0.5M calcium chloride to the beaker. Stir the contents of
the beaker with a stirring rod.
11. To ensure complete precipitation, add another 5ml of
0.5M calcium chloride to the beaker, and stir again.
12. Measure and record the mass of a piece of filter paper.
13. Fold the filter paper in half, and then fold it in half again.
14. Open one side as shown, to create a funnel to collect the precipitate.
15. Place the paper funnel into a glass or plastic funnel. Moisten the paper
with distilled water from your wash bottle. Press and mold the filter
paper with your fingers until it conforms to the walls of the funnel.
16. Place the lined-paper funnel in the ring on the ring stand. Place an Erlenmeyer
flask under the funnel to catch the filtrate as it passes through the filter paper..
17. Stir the solution to suspend the carbonate precipitate and then pour
solution against the stirring rod, allowing the liquid and precipitate
to flow down the rod onto the filter paper, as shown. Caution: Don’t
allow the liquid to rise above the top edge of the filter paper.
18. Swirl the last bit of the liquid to re-suspend the precipitate and then pour it against the stirring rod into the funnel.
19. Rinse any remaining precipitate from the walls of the beaker using a wash bottle. Also rinse the stirring rod,
allowing all the washings to flow onto the filter paper.
20. After all the liquid has drained from the precipitate, rinse the precipitate on the paper with a few mLs
of distilled water. Allow the funnel to completely drain.
21. Obtain a dry paper towel. Use a marker to label it as your lab group number.
22. Carefully pull back the top edges of the filter paper from the walls of the funnel. Gently lift the filter paper from
the funnel and place it on the paper towel.
23. allow the filter paper to dry over night. Mass the filter paper and unknown, and record in the Data Table.
24. Exchange data with a group that worked with the other unknown.
25. Complete the Data Table and determine the identity of Unknown A and Unknown B.
Data Table
Unknown A
1. Mass of crucible
2. Initial mass of crucible + X2CO3
3. Dried mass of crucible + X2CO3
4. Mass of X2CO3
5. Mass of filter paper
6. Mass of filter paper + CaCO3
7. Mass of CaCO3
8. Moles of CaCO3
9. Experimental formula mass of X2CO3
10. Atomic mass of X in X2CO3
11. Closest atomic mass in Group 1 metals
12. Theoretical formula mass of compound
13. Percent Error
***Skip***
Unknown B
***Skip***
(exp 𝑚𝑎𝑠𝑠−𝑡ℎ𝑒𝑜𝑟 𝑚𝑎𝑠𝑠)
𝑥100%
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑎𝑠𝑠
Calculations Data Table steps 8 - 13 require you to show your work, for both unknowns.
Conclusions
1. Why was it necessary to heat the unknown carbonate before dissolving it in water.
2. The official lab procedure requires you to heat the carbonates in steps 4-5 multiple times, re-massing each time,
until you reach a constant mass. We didn’t utilize this critical step because it eats up a lot of time,
not allowing us to finish the lab. Also, the official lab wanted us to heat the filter paper in a drying oven
rather than leaving it out over night. Why?
3. How would your atomic mass prediction have been affected if not all the water had been evaporated from
the calcium carbonate precipitate?
4. Why was it necessary to have ‘excess’ Ca2+ ions to precipitate the unknown carbonates?
Lab3.2 Gravimetric Analysis of a Carbonate
Name____________
Prelab questions: An unknown metal carbonate, X2CO3, was evaluated, and the following data recorded:
1. Mass of crucible
8.30 g
Calculations (Show work rows 8 - 13)
2. Initial mass of crucible + X2CO3
9.34 g
3. Dried mass of crucible + X2CO3
9.31 g
4. Mass of X2CO3
5. Mass of filter paper
1.00 g
6. Mass of filter paper + CaCO3
1.69 g
7. Mass of CaCO3
8. Moles of CaCO3
9. Experimental formula mass of X2CO3
10. Atomic mass of X in X2CO3
11. Closest atomic mass in Group 1 metals
12. Theoretical formula mass of compound
13. Percent Error
1. What is the experimental formula mass of X2CO3? (Fill in row 4)
2. Determine the identity of the Group 1 metal X. (Fill in rows 7 – 12)
3. Calculate the % error for the exp formula mass. (Fill in row 13)
Objectives:
Procedure:
Data Table
Unknown A
Unknown B
1. Mass of crucible
2. Initial mass of crucible + X2CO3
3. Dried mass of crucible + X2CO3
4. Mass of X2CO3
5. Mass of filter paper
6. Mass of filter paper + CaCO3
7. Mass of CaCO3
8. Moles of CaCO3
9. Experimental formula mass of X2CO3
10. Atomic mass of X in X2CO3
11. Closest atomic mass in Group 1 metals
12. Theoretical formula mass of compound
13. Percent Error
Calculations Data Table steps 8 - 13 require you to show your work, for both unknowns.
8.
9.
10.
11.
12.
(exp 𝑚𝑎𝑠𝑠−𝑡ℎ𝑒𝑜𝑟 𝑚𝑎𝑠𝑠)
13. 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑚𝑎𝑠𝑠 𝑥100%
Conclusions
1. Why was it necessary to heat the unknown carbonate before dissolving it in water.
2. The official lab procedure requires you to heat the carbonates in steps 4-5 multiple times, re-massing each time,
until you reach a constant mass. We didn’t utilize this critical step because it eats up a lot of time,
not allowing us to finish the lab. Also, the official lab wanted us to heat the filter paper in a drying oven
rather than leaving it out over night. Why?
3. How would your atomic mass prediction have been affected if not all the water had been evaporated from
the calcium carbonate precipitate?
4. Why was it necessary to have ‘excess’ Ca2+ ions to precipitate the unknown carbonates?