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Title:        Molecular Weight of a Volatile Liquid

Purpose:    To determine the molecular weight of methanol.

Procedure:

Get 2 aluminum foil squares (approximately 2x2 inches) and make into tightly fitting caps over a clean, dry, 250-mL Erlenmeyer flask.  Put a pin hole through one of the aluminum caps.  Mass the flask with both caps on it.  Add 5 mL of unknown liquid to the flask.  Clamp the flask (with the pin-hole aluminum cap only) into a water bath (approx. 70 C) and heat until the liquid completely disappears.  Continue to heat for 3-5 minutes after the liquid disappears.  Measure the temperature of the water bath and the barometric pressure.  Remove the e flask and dry the outside thoroughly.  Let the flask cool for 4-5 minutes.  The liquid will condense and cause a lowering of pressure in the flask.  Air will come back inside the flask.  Put the second foil cap on top of the pin-hole cap and mass.  Measure the volume of the flask and repeat the procedure for  a second trial.

Questions:

1.    Why don't you put the second aluminum cap on the flask before it cools off?

2.    Why is it necessary to let air back into the flask while it is cooling?

3.    What is the purpose of the second aluminum cap?