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To determine the molecular weight of a volatile liquid.


[This laboratory exercise was adapted from one in Beran and Brady's Laboratory Manual for General Chemistry , John Wiley & Sons, 1982]

The Dumas Method for determining the molecular weight of a volatile liquid, named after John Dumas (1800-1884) requires the use of the ideal gas law (). The accuracy of the method is therefore dependant on how well the vapors of the volatile liquid emulate an ideal gas at the experimental contitions.

In this experiment a liquid will be vaporized at a measured temperature, T, into the measured volume, V, of an Erlenmeyer flask. After the barometric pressure, P, is recorded, the mols of gas, n, are calculated from the equation. The mass difference between an empty and gas filled flask allows us to calculate the mass of the gas. The molecular weight (MW) is then calculated by the equation: . Alternatively one can substitute for n in the ideal gas law to obtain the following equation:


You are to complete two trials in this experiment and post them on the chalkboard for use by the entire class in answering some of the questions.

1. Determine the total mass of a 125 mL Erlenmeyer flask, a rubber band, and a square of aluminum foil.

2. Accurately measure the volume of the Erlenmeyer flask by totally filling the flask with water and transferring the water to a graduated cylinder. (Note: this step can be done at the conclusion of the experiment.)

3. Create a hot water bath by filling a 400 mL beaker half full of water. Heat to boiling. While waiting for the water to boil pour about 5-6 mL of unknown liquid into the flask. Secure the aluminum foil over the mouth of the flask with the rubber band. Poke a small hole in the foil with a pin to let excess vapor escape during heating.

4. Clamp the flask assembly into the beaker so that flask is as far down as possible in the beaker. Heat at the boiling point of water until liquid is no longer visible in the flask, continue heating for another 10 minutes. Record the boiling point of water to the nearest ±0.1 oC. Also, record the current barometric pressure.

5. Remove the flask and allow it cool to room temperature. Dry the outside of the flask and mass it along with its contents, the aluminum foil and rubber band.

6. Repeat for the second trial.

Would the molar mass of a volatile liquid, calculated using the procedure in this experiment, be incorrectly high, incorrectly low, or unaffected by the following procedural changes.

a. you did not completely vaporize the liquid when you heated it

b. the foil cap got wet while you were cooling the flask and its contents with running water.

c. you added the boiling stone to the flask after you had already determined the mass of the empty flask and foil cap

d. You forgot to measure the volume of the flask, so you used the volume pronted on the flask for your calculations.

e. your unknown liquid had a boiling point of 102.3 degrees C

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