Introduction

An important application of the Ideal Gas Equation (PV = nRT) is the experimental determination of the molecular weights of gases and volatile liquids. In this experiment you will heat an unknown volatile liquid above its boiling point, but to a known temperature, and collect a known volume of its vapor at this temperature and the known barometric pressure. You will then cool the vapor (which will condense back to a liquid) and determine its weight.

The number of moles of any substance equals the mass of that substance divided by its molar mass. Substituting this relationship,  into the Ideal Gas Equation, you will have which can be rearranged to give you the relationship,

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Equipment
 

125mL Erlenmeyer flask	6 x 6 cm and 8 x 8 cm squares of aluminum foil
copper wire (about 15 cm long)	10mL graduated cylinder
rubber band	Pin
boiling chips	600mL beaker
watch glass	hot plate
250mL graduated cylinder	Thermometer
crucible tongs	analytical balance

Chemicals

student: Unknowns (Flammable)

Disposal Spill/Disposal: F, Unknown volatile liquid. * Caution these chemicals may be flammable and/or a Health Hazard*

Procedure

1. Weigh a clean, dry 125mL Erlenmeyer flask together with a 6 x 6cm square of aluminum foil and the copper wire on an analytical balance to 0.0005 g. Record the weight on the data sheet.

2. Use a clean calibrated dropper to measure out 5 mL of an unknown volatile liquid into the 125mL flask. Be sure to record the number or letter of your unknown. Place the 6 x 6 cm square of aluminum foil over the mouth of the flask, crimp it down, and secure it in place using the copper wire that is about 15 cm long.

3. Place a second piece of aluminum foil (8x 8 cm) over the first foil and secure it in place using a rubber band. This outer cover helps to prevent water from collecting under the folds of the inner aluminum foil. Using a pin, poke a very small hole through both aluminum covers.

4. Set up the apparatus, as shown, underneath one of the "elephant trunk" hoods in the lab. Put about 150mL of water along with four or five boiling chips in the 600 or 1000mL beaker. Place the flask in the beaker and cover the beaker with a watch glass. In some cases the watch glass may have to be inverted and loosely taped down. Heat the water "very gently" so that it takes ten minutes or more for it to boil. Continue heating until all of the volatile liquid has vaporized. Mentally make note of this time and heat for another two minutes. Avoid breathing the vapors of the volatile compound. Record the temperature of the boiling water.

5. Stop heating the water. With crucible tongs or a paper towel remove the watch glass from the top of the beaker and carefully remove the Erlenmeyer flask from the water. Dry the outside of the flask with a paper towel. Be sure to dry very well around the aluminum foil. Remove the outer foil and dry the inner foil.

6. Allow the flask, with its inner foil and copper wire, to cool to room temperature. This will take about ten minutes. When the flask is cool, reweigh it (with aluminum foil and copper wire still in place). Record the weight on the data sheet.

7. Record the barometric pressure, which the instructor will measure.

8. Carefully remove the copper wire and aluminum foil. Rinse out the flask with tap water being careful not to get the residual unknown volatile liquid on your hands. Fill the flask to the top with tap water and level off the water with the piece of aluminum foil. Using a 250mL graduated cylinder, measure the amount of water that was present in the flask. This is the volume of the flask and, also, the volume occupied by the vapor of the volatile liquid.

Disposal

A designated hazardous waste bottle will be available for the remaining unknown - labeled as MW of a Volatile Liquid "Hazardous Waste".