MKHS Chemistry 1A
Lab partners ________________________________________________________
Density of CO2 from Alka Seltzer
We will collect the gas by “water displacement” in a graduated cylinder and can then easily measure its volume. We
can also measure the temperature (the same as water temperature) and the pressure. However, the pressure in the
graduate will be equal to the atmospheric pressure but is not due solely to the carbon dioxide; there will be a small
amount of water vapor that will also contribute to the pressure. According to Dalton’s law of partial pressures,
Ptotal = PCO2 + Pwater. The partial pressure of the water depends on the temperature and can be looked up in a table.
Thus, knowing Ptotal and Pwater, one can calculate PCO2 and then use this value to calculate V.
RECORD ALL DATA IN A DATA TABLE.
Procedure:
1. To a 250 Erlenmeyer flask, add 20 mL of 2M HCl solution.
2. Find the mass of the flask + 2M HCl acid.
3. Obtain a tablet of Alka seltzer and record the mass.
4. Connect rubber tubing to the overflow stem of the trough leading to the sink. Connect the rubber stopper with
the glass elbow and tubing to the lower stem where the gas will enter.
5. Fill the trough ¾ full with water.
6. Fill a 1000ml graduated cylinder with ~900mL of water.
7. Invert in the trough, under the water, making sure that you minimize the air bubbles getting into the cylinder.
Record the volume of in the graduated cylinder.
8. When you are ready - have one person add the complete Alka-seltzer tablet to the flask and quickly cap it with
the stopper (+ hose connected). Wait for the change and gas generation to proceed. *You may need to swirl
the flask to assure the change/gas generation has completed.
9. Keeping the graduated cylinder inverted, read the volume of gas collected. Record.
10. Mass the Erlenmeyer flask with remaining contents. Record.
11. Repeat experiment.
12. Return your materials. Clean your station. Wash any waste down the sink.
RESULTS
1. Use Dalton’s Law to correct for the pressure of the CO 2 gas collected using the formula:
Ptotal = PCO2 + Pwater +Pair. Calculate the Pair in the graduate based on the atmospheric pressure and percent of
the total gas you collected. You will need to look up the partial pressure of water vapor at the temperature you ran
the experiment (water temperature). Once you have corrected for the pressure, calculate the volume using the
ideal gas law equation. Calculate n based on the mass loss in the reaction flask. The total pressure is equal to
atmospheric pressure (obtain from barometer).
Experimental Volume of CO2 ____________
2. Obtain the mass of sodium bicarbonate in one tablet of Alka Seltzer. Calculate the theoretical volume of carbon
dioxide produced from an Alka Seltzer tablet using stoichiometry. Show your work. The equation for the reaction
of the sodium bicarbonate and acid is: HCl(aq) + NaHCO3(aq)  H2O(l) + CO2(g) + NaCl(aq)
3. Experimental Volume of CO2 ____________
Theoretical Volume of CO2 _____________
% Error _________
4. Calculate the density of carbon dioxide gas at STP. Compare to the accepted value.
Accepted value for density ___________
Experimental density_____________
5. Discuss possible reasons why your results do not match the theoretical values.
% Error _______________