Objectives

To determine the molecular weight of a volatile liquid.

Discussion

[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:

Procedure

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

a. If the liquid is not completely vaporized when heated, the calculated molar mass of the volatile liquid would be incorrectly low. This is because the molar mass is calculated based on the mass of the gas formed when the liquid is completely vaporized. If not all of the liquid is vaporized, the measured mass would be lower than it should be, resulting in a lower calculated molar mass.

b. If the foil cap got wet while cooling the flask and its contents with running water, the molar mass of the volatile liquid would be unaffected. The wet foil cap does not impact the mass of the gas, which is the key parameter used to calculate the molar mass.

c. If the boiling stone is added to the flask after determining the mass of the empty flask and foil cap, the molar mass of the volatile liquid would be unaffected. The boiling stone does not contribute to the mass of the gas, so it does not affect the calculation of the molar mass.

d. If the volume of the flask is not measured and the printed volume on the flask is used for calculations, the molar mass of the volatile liquid would be incorrectly high or low, depending on the accuracy of the printed volume. If the printed volume is accurate, then the molar mass calculation would not be significantly affected. However, if the printed volume is not accurate, it would introduce errors into the calculation, resulting in an incorrectly high or low molar mass.

e. If the unknown liquid has a boiling point of 102.3 degrees Celsius, it would not directly affect the molar mass calculation in this experiment. The molar mass is determined based on the mass of the gas formed when the liquid is vaporized, regardless of its boiling point. However, it's worth noting that the accuracy of the boiling point measurement is important for obtaining accurate results in the experiment overall.