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Determination of Molecular Formula: Dumas Method


Using the Combined Gas Law, and the molar volume of a gas at STP, determine the molecular weight of a volatile liquid. Then, using the Universal Gas Law, again determine the molecular weight. Compare the results of the two calculations. Finally, from the molecular weight and the empirical formula supplied by your instructor, write the molecular formula.


250ml Erlenmeyer flask, aluminum foil to cover the top of the flask (a circle approx. 2cm in diameter), needle, centigram balance, 1000mL beaker, ring stand, iron ring, wire gauze, burner and tubing, large clamp, barometer, thermometer, and pipet.


5mL of unknown volatile liquid.


The flask contains air that is exactly the volume of the flask and has mass. When the flask is heated with the volatile liquid inside evaporates and expands to the size of the flask and forces all of the air out of the flask; the excess vapor will escape out of the pinhole as well. The temperature of the vapor in the flask is determined from the temperature of the boiling water. The pressure can be ascertained by using the barometer. When the flask is cooled the vapor in the flask will cool and condense and air that is almost equal to the original volume will be drawn back into the flask. The volume of the flask can be determined after the lab by washing the flask and using a graduated cylinder. Thus, any change in the mass of the flask after condensation of the vapor compared to the "empty" flask is due to the mass of the vapor that was in the flask at the temperature of the boiling water. From the temperature, pressure, volume, and mass of vapor the molecular weight of the unknown liquid can be calculated by the methods mentioned earlier in the objectives of the lab. Using the molecular weight and the empirical formula the molecular weight can be determined.


  1. Place 750 mL of water in the 1000mL beaker and begin heating.
  2. Determine the mass of the 250mL Erlenmeyer flasks and the aluminum foil used to cover the top.
  3. Pipet 5 mL of the unknown liquid into the flask.
  4. Cover the flask with foil and fold down around the neck and trim to fit securely.
  5. With the needle punch a SMALL HOLE in the center of the aluminum foil.
  6. Using the large clamp, clamp the neck of the flask as close to the top as possible.
  7. Attach the clamp to the ring stand so that the flask is as far in the water as possible.
  8. Immediately add or remove water from the 1000mL beaker to keep the water level within a centimeter of the top of the beaker.
  9. Bring the water to a boil and maintain it for 7 minutes.
  10. Determine the temperature of the boiling water.
  11. Determine the atmospheric pressure using the barometer.
  12. After 7 minutes have passed, remove the flask from the boiling water and cool by running cool water over the base of the flask. CAUTION!!! Do not get water on the foil while running under the water since it will be drawn into the flask and give a false reading on the mass.
  13. Dry the flask with a towel and determine the mass of the flask and the foil and the condensed vapor.
  14. Determine the mass of the vapor?
  15. Using the graduated cylinder and water determine the volume needed to fill the flask to the top.


  1. Mass of flask, foil, and condensed vapor (after heating): 78.942g
  2. Mass of flask and foil: 78.426g
  3. Mass of Vapor: 0.516g
  4. Volume of vapor = volume of flask: 274.5mL
  5. Temperature of vapor: 370.0K
  6. Pressure of vapor = barometric pressure: 651.5torr
  7. Volume vapor would occupy at STP: 274.5mL
  8. Number of moles of vapor: 0.0078625moles
  9. Mass of one mole (molecular weight): 0.066616 g/mol
  10. Mass of one mole from original data: 0.065609 g/mol: using the Universal Gas Law
  11. Empirical Formula (given): CH
  12. Molecular Formula:C6H6