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A COMPARISON OF THE SOLUBILITIES OF CARBON DIOXIDE IN WATER AT VARIOUS TEMPERATURES

* PURPOSE

By making measurements on a sample of carbon dioxide, students are able to determine the molar volume of CO2. They arealso introduced to the concept of buoyancy and its importance when massing objects whose masses are small compared to their volumes.

* DESCRIPTION

This experiment is appropriate for a general or first-year college-prep course. Using Seltzer water samples at various temperatures, students determine how the solubility of carbon dioxide in water varies with temperature. The-acid, formed when carbon dioxide reacts with water, is titrated with sodium hydroxide solution to a phenolphthalein endpoint.

* TIME REQUIRED

One lab period.

* MATERIALS

Chemicals:
Seltzer water
2.0 M NaOH solution (80 g NaOH dissolved in enough distilled or deionized water to make 1.0 liter of solution)*
phenolphthalein solution (1% in ethanol)*
Equipment:
1-L beakers*
thermometers
ice bath and warm water bath - large enough to hold a 1-L beaker
100-mL graduated cylinders
200-mL Erlenmeyer flasks
eye dropper or Pasteur pipet*
*See Modifications / Substitutions.

* HAZARDS

The NaOH solution is caustic; avoid contact with skin. Goggles must be worn throughout the experiment.

* MODIFICATIONS/SUBSTITUTIONS

  1. Sodium hydroxide is available as lye in grocery stores.
  2. Phenolphthalein indicator may be prepared by dissolving crushed Ex-Lax tablets from a drugstore in ethanol and filtering. Ethanol is available as rubbing alcohol, but isopropyl alcohol is also sold as rubbing alcohol.
  3. Any large jar or other container could be substituted for the 1-L beakers.

* PROCEDURE

Sample preparation by teacher
  1. Prior to lab session, prepare samples by placing unopened bottles of Seltzer water in an ice-water bath until samples are at temperature of bath.
  2. About 0.5 hour before students are to begin experiment, open the necessary number of bottles of Seltzer water. Mix the contents, divide into three 1-L beakers, and replace one beaker in the ice-water bath. Allow one beaker of Seltzer to stand at room temperature and place the other in a warm water bath.
Experimental procedure for students
  1. Measure out 50 mL of Seltzer water and transfer to an Erlenmeyer flask. Add 3 drops phenolphthalein solution and swirl gently for one minute to remove any remaining mechanically trapped carbon dioxide.
  2. Using a dropper, titrate with 2.0 M NaOH solution until the pink phenolphthalein color persists for 30 seconds. Record the number of drops of NaOH solution used.
  3. Repeat steps 1 and 2 using samples at different temperatures.
  4. Multiple samples may be titrated if time permits.
  5. Plot a graph of drops of NaOH solution required for neutralization vs. temperature and draw conclusion about how the solubility of carbon dioxide varies with temperature.

* DISPOSAL

Titrated samples may be disposed of by flushing down the drain with running water.

* DISCUSSION

Carbonated beverages are bottled under a carbon dioxide pressure slightly greater than 1 atmosphere. When the bottles are opened to the air, the partial pressure of CO2 above the solution is decreased and CO2 bubbles out of the solution. The bottles should be cold when opened because CO2 is most soluble at low temperatures. Bottles should be opened prior to doing the experiment to allow the dissolved CO2 to reach equilibrium with the lower pressure of CO2 in the air. Preparing the samples early also allows each sample to reach temperature equilibrium. When carbon dioxide dissolves in water it reacts with water to form an acidic solution which can be neutralized by the addition of base.
CO2 (aq) + H2O (l) <====> H+(aq) + HCO3-(aq) [sometimes written as H2CO3 (aq)]
Results obtained by this procedure are intended to indicate a trend in the solubility of the carbon dioxide as a function of temperature. They will not agree with literature values for the solubility of carbon dioxide which are usually measured at higher partial pressures of the gas.

* TIPS

  1. Although the procedure is rather straight forward, it is necessary to be careful to provide samples that vary only in their temperature and that students use reproducible methods in doing the experiment. Best results are obtained when the dropper is held vertically while titrating.
  2. The pre-lab preparation should include discussion of how students can standardize their methods and may include some discussion of how the samples are prepared to eliminate variables.

* REFERENCES

Brown, T.L. and LeMay, H.E., Jr., Chemistry - The Central Science, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981. Chaps. 12, 15, and 17 discuss the chemistry of solutions of carbon dioxide in water.

Gordon, G. and Keifer, The Delicate Balance, Harper & Row, Publishers, New York, 1980, pp. 110-111. In an experiment described here, bottles of Seltzer water maintained at three temperatures are opened and the acid (from dissolved CO2) is titrated using burets.

Submitted by Eva Lou Apel, Michael Bannon, Joseph Baron, John Brodemus, and Elna Clevenger


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