Molecular Weight of a Volatile Liquid

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 (PV=nRTÐ). The accuracy of the method is therefore dependent on how well the vapors of the volatile liquid emulate an ideal gas at the experimental conditions.

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: Ð. 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.

Questions
1. Determine the molecular weight of the unknown for each trial. Show all pertinent calculations for trial 1.

2. If the outside of the flask is not dried after vaporizing the liquid, will the unknown's calculated molecular weight be too high or too low? Explain?

3. Suppose the atmospheric pressure is assumed to be 1 atmosphere instead of its actual value. How will this error affect the molecular weight of the unknown? Explain.

4. What is the percent error for the molecular weight if the false assumption in question 3 is made? Show your work.

5. If the vapor's volume is assumed to be 125 mL instead of the measured volume, what is the percent error of the unknown's calculated molecular weight? Show your work.

6. If all of the unknown does not vaporize in the 125 mL Erlenmeyer flask, will the reported molecular weight be too high or low? Explain.

7. Suppose the thermometer is miscalibrated and reads 0.2 degree Celcius higher over its entire temperature range than actual. Does this affect the unknown's molecular weight? Explain.

8. An inherent error in determining the unknown's molecular weight is the design of the apparatus: the boiling water bath does not envelop the upper part of the Erlenmeyer flask. What can you do to minimize the error? How does this error affect the unknown's reported molecular weight?. Explain.

9. Determine the class average and the standard deviation.

10. Obtain the identity of the unknown from your instructor and determine your relative error.

11. The Van der Waals equation (given below) corrects the Ideal Gas Equation for the volume of the gas particles (b) and the intermolecular forces of attraction between the particles (a). Using the density of the liquid and its molecular weight, calculate an approximate value for b. With this value of b and your data solve the Van der Waals equation for the variable a,. What units will be attached to both a and b.

12. From the class values listed on the chalkboard for the molecular weight of the unknown liquid determine which ones would be considered outliers at the 96% confidence level and rejected. After rejecting these values calculate the class average for the and the standard deviation.


Teacher Notes

The Dumas method for the molecular weight determination of a volatile liquid has appeared in many laboratory laboratory manuals. My contribution to these labs is question 11.

Care must be taken in the selection of the liquid to be used. Avoid selecting one that is flammable. If flammable liquids are used (and I have used them on occasion) make sure that students are informed of the dangers and do not use excessive amounts.


 
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