Molecular Mass of a Volatile Liquid
Introduction
The ideal gas law may be
expressed as
PV =
nRT (1)
where P = gas pressure in
atmospheres
V = gas
volume in liters
n =
number of moles of gas
R = gas
constant
T = gas
temperature in degrees Kelvin
At
standard temperature (0°C or 273 K) and pressure (1 atm or 760 torr) one mole
of any gas occupies 22.4L. With this
information it is possible to determine that R (the gas constant) has a value
of 0.0821 L·atm/mol· K.
The number of moles of a gas
is equivalent to the number of grams of gas divided by the molecular mass of
the gas.
(2)
Combining expressions (1)
and (2) it is possible to relate pressure, temperature, volume and the number
of grams of a gas to the molecular mass of the gas.
(3)
Solving this expression for
the molecular mass we have
(4)
In
this experiment a volatile liquid will be heated in a water bath at atmospheric
pressure until only vapor remains. The
vapor that does remain in the reaction vessel will be weighed and once the
volume of the reaction vessel has been measured it is possible to calculate the
molecular mass of the volatile liquid.
Safety
The
vapors of most organic liquids are toxic and therefore, care should be
exercised during handling. Do not
breathe the vapors or allow the unknown liquids to come in contact with the
skin. If any contact occurs, flush area
with copious amounts of water.
Keep
liquids away from open flames. Most
organic substances are flammable.
Procedure
1)
Fill a pneumatic trough about 3/4 full of tap water and set it aside to allow
it to reach room temperature. The trough is to be used in step # to cool the
reaction system to room temperature.
2)
Obtain from the stockroom a special round-bottom flask, stopper and rubber
weighing ring and an unknown liquid. Be
sure to record the number of your unknown.
3)
Support the round-bottom flask with its stopper on the rubber weighing
ring. Weigh the empty dry flask,
stopper and rubber ring on the Mettler Balance. Record the total mass on the data sheet.
4) Place half of your unknown liquid (about 5 mL)
into the round-bottom flask and insert the stopper. Assemble the apparatus as shown to the lower right. 
5)
Heat the water in the 600 mL beaker slowly. Do not let the water boil. Measure and
record the temperature of the water.
6) The liquid level in the round bottom flask should gradually drop as vapor escapes through the stopper. When the liquid has disappeared from the flask (make sure none remains in the hole of the stopper), remove the flask from the boiling water and clamp it in an upright position in the pneumatic trough.
7) Allow it to cool to room temperature. (Check after one to two minutes. At this point, the unknown vapor in the flask would have condensed back to the liquid state.) Then remove the flask from the trough and dry the outside very carefully.
8) Reweigh the flask, stopper and rubber ring and record on the data sheet.
9)
Repeat the procedure using the other half of your unknown liquid. Do not wash the flask.
10) After the second trial the volume of the flask may be determined as follows: Fill the flask with water nearly to the top. Place the stopper in the flask. A little water should spurt out through the stopper. Wipe off the excess, then lift out the stopper carefully so that the small amount of water in the hole of the stopper drains back into the flask. Measure the amount of water contained in the flask using a graduated cylinder.
11)
Get the barometer pressure reading and perform necessary correction calculation
12)
Once the experiment has been completed, return the round-bottom flask, stopper
and rubber weighing ring to where you obtained them.