MOLECULAR WEIGHT
of a volatile liquid by the
DUMAS METHOD
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You will weigh a dry 250 mL erlenmeyer flask with an aluminum foil cap using a mechanical Analytical Balance. Make the cap from an ordinary piece of aluminum foil, trimmed to fit snugly across the top and down the side about a 1 cm or less. Using a pin, make a pin hole in the cap to insure that the vapors will escape during the heating process.
To this weighed flask add about 5 mL of your UNKNOWN.
This volume is not critical.
But, if you add too little the vapors will not completely displace the air. And, if you add too much it will take much longer time to evaporate all the liquid. Also impurities can have a bigger effect the more UNKNOWN you use.
...
  For this experiment the UNKNOWN must:
  • Be a liquid (with "low" vapor pressure) at room temperature
  • Boil below 100 șC
  • Obey the Ideal Gas Law as a vapor

The reaction mixture is heated in a boiling water bath.  The liquid in the flask will boil and displace the air.
CAUTION: Boiling water can scald you. Remember to use a guard ring clamp (not shown) to prevent the beaker of boiling water from falling!!!

After all the liquid has boiled off, continue heating for an additional 2-4 minutes to insure the temperature of the vapor in the flask is the same as that of the boiling water. Take the temperature of the boiling water.

Quickly cool the flask under running cold water being careful not to get water inside the flask and not to cool the flask below the dew point.
Quickly dry the outside of the flask.
Quickly weight the flask on the Electronic Analytical Balance.
As you do this, try to understand why
"Quickly" was used three times above!

With a little care molecular weights within 5% can be obtained.

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WARNING

You should be aware of two possible "accidents".

  1. Some Unknowns are flammable and the open flame can ignite them.  If this happens, don't panic.  It will rapidly burn itself out.  Notify the instructor immediately.  The trial will have to be repeated. This is a very rare occurance.  Keep the flame heating the water as low as possible, yet the water must be at a slow boil.

  2. Careless handling of the apparatus can cause the 600 mL beaker with boiling water to crash down on the bench top and break.  The boiling water can scald you! Always keep the Beaker with water solidly supported in the middle of the ring clamp.  Use common sense to prevent this "accident ". 
    We currently use a large ring clamp to hold the top of the beaker of water in place.

©  RW Kluiber  11/12/98