Lab 9 – Carbonate Analysis Via Gas Evolution
Introduction:
One characteristic of all Group I and Group II carbonates is that one mole of carbon dioxide is released when
one mole of the carbonate reacts with an acid. For example, the reaction between calcium carbonate, CaCO3,
and hydrochloric acid, HCl, produces water, H2O, and carbon dioxide, CO2, according to:
CaCO3 (s) + 2HCl  Ca2+ (aq) + 2Cl- (aq) + H2O (l) + CO2 (g)
The stoichiometry of the balanced reaction shows that one mole CO2 is produced for every mole of carbonate
salt reacted. To ensure that all of the carbonate is converted to carbon dioxide, an excess amount of acid is
usually used.
The one – to – one correspondence can be exploited to identify a Group I or Group II carbonate. This is due to
the fact that one mole, or 44.01 g, of CO2 will be evolved from one mole of carbonate. Since the molar mass of
each carbonate is different, the mass percentage of CO2 will be unique. In the calcium carbonate example given
above, 100.0872 g of CaCO3 will yield 44.01 g of CO2. This means the mass percentage of CO2 in this
carbonate is:
44.01 g  mol -1
 100%  43.9715%
100.0872 g  mol -1
In this experiment, you will be able to determine the mass of the unknown carbonate by difference with a
balance. The mass of evolved carbon dioxide will be determined by measuring its volume, temperature, and
pressure. The CO2 will be 
bubbled into an inverted graduated cylinder that is initially filled with water. As the
gas enters the cylinder, it displaces the water. To measure the volume of the gas trapped in the cylinder, V, you
move the cylinder up and down until the meniscus is at the same height as the surrounding water bath. At this
point, the pressure of the gas inside the tube is the same as the air pressure in the lab. The total pressure,
however, is made up of two components: CO2 and H2O. Water is present in the gas phase due to the fact that it
has a relatively high vapor pressure, even at room temperature. According to Dalton’s Law of Partial Pressures,
the pressure of CO2, P, may be determined if the total pressure of the cylinder, Ptotal, and the water vapor
pressure, Pwv, are known:
P = Ptotal – Pwv
With the temperature of water, T, known, the ideal gas law may be used to calculate the moles of carbon
dioxide, n, trapped in the cylinder:
PV
n
RT
From this, the mass of CO2 used can be determined, thus, allowing us to solve for the percent CO2 in the
original carbonate.

Pre – Lab
1. Make a list of the materials and chemicals that you will need to use during today’s lab
2. Make a table for all the data that you will collect for the lab.
3. Determine the percent by mass of CO2 for barium carbonate.
Materials:
Equipment
Beaker, 600 mL
Orange Bucket with water
Gas Transfer Assembly
Hydrochloric Acid, 6 M
Quantity
1
1
1
10 mL
Equipment
Graduated Cylinder, 50 mL
Erlenmeyer Flask, 250 mL
Thermometer
Unknown Carbonate
Quantity
1
1
1
1g
Procedure:
1.Obtain an unknown carbonate sample and mass out at least 1 g of substance. You will be performing 3 trials,
so you need three masses that are around 1 g. Be sure to record the exact value that you obtain.
2. Take one of your 1 g carbonate samples and put it in a clean and dry 250 mL Erlenmeyer flask.
3. In your data table, record the atmospheric pressure that is written on the board.
4. Measure the temperature of the water in your orange bucket. Record the value in your data table.
5. Submerge the graduated cylinder in the water in your orange bucket. Fill the graduated cylinder with water
and invert it. Check to ensure that there are no bubbles inside the graduated cylinders. If there is an air bubble in
the graduated cylinder, dump the water out and fill it out again. Do this until no air bubbles are inside the
graduated cylinder.
6. Fill a small test tube ¾ of the way with 6 M HCl. Be sure that it is not so full that some of the HCl will spill
out.
7. Gently, slide the small test tube into the Erlenmeyer. Make sure no HCl spills out.
8. Attach a balloon over the top of the Erlenmeyer. Take the balloon and cut the top off. Tape the cut off end to
one end of the plastic tubing. Slide the other end of the plastic tubing into the orange water bucket and insert the
end under the graduated cylinder.
9. Once the plastic tubing is safely attached to each end, shake the Erlenmeyer until the HCl spills out to react
with the carbonate. Keep shaking until all of the carbonate has been reacted and all of the CO2 has been caught
in the graduated cylinder.
10. Move the graduated cylinder up and down until the water level inside the graduated cylinder is at the same
level. Measure the volume of gas that is contained in the graduated cylinder.
11. Repeat the procedure with the two other samples of the unknown carbonate.
Data:
Calculations: Due Monday/Tuesday!
1. Calculate the partial pressure of carbon dioxide that was evolved:
P = Patm – PH2O
For the partial pressure of H2O, use the table that is on the board.
2. Calculate the moles of CO2 evolved.
3. Calculate the mass of CO2 evolved.
4. Calculate the mass percent of carbon dioxide that is in the unknown sample and then use the table below to
identify the unknown sample.
Carbonate
Percent CO2
CaCO3
43.97
K2CO3
31.84
Li2CO3
59.56
MgCO3
52.20
Na2CO3
41.52
Analysis: Due Monday/Tuesday!
1. In a well-thought-out paragraph, address the following discussion point: One possible source of error in the
experiment is the presence of air bubbles in the inverted graduated cylinder or perhaps air that is trapped in the
gas transfer assembly that happens to make its way to the cylinder before the carbon dioxide is transferred
through it. How would the presence of this air affect the results?
2. Carbon dioxide is known to dissolve readily in water by the following reaction:
CO2 + H2O  HCO3- + H+
How might this impact the final results that you obtained?