Chapter 14 Laboratory: Gas Chemistry 265CUTIOA nS
Although only a small amount is used in this lab, acetone
is extremely flammable. Be careful not to expose acetone
liquid or vapor to an open flame. Use an exhaust hood, or
work outdoors or in a well-ventilated area. Water at 50°C
is hot enough to burn you badly. Wear splash goggles,
gloves, and protective clothing.z
We use a simpler apparatus, shown in Figure 14-7, to determine
the molar mass of acetone by the Dumas method. First, we
accurately determine the mass of an empty Erlenmeyer flask and
a piece of aluminum foil used to stopper it. We then introduce
a small amount of liquid acetone and immerse the flask in a
warm water bath to boil the acetone, converting it to vapor. As
the acetone vaporizes, it displaces the air in the flask through a
small pinhole in the aluminum foil. When all of the acetone has
vaporized, we record the temperature (T), and then cool the flask,
causing the vapor to condense to liquid acetone. We reweigh the
flask to determine by difference the mass of acetone vapor it
contained. We then determine the volume (V) of the flask, and
the atmospheric pressure (P). Using those data, we calculate the
molar mass of acetone.
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FIGURE 14-7:
My apparatus for determining molar mass from vapor density
POCEDURER
1.f you have not already done so, put on your splash I
goggles, gloves, and protective clothing.
- Loosely crimp a piece of aluminum foil around the
mouth of the flask. - Weigh the flask and aluminum foil, and record the mass
to 0.01 g on line A of Table 14-5. - Transfer about 5 mL of acetone to the flask. The exact
amount is not critical. - Crimp the aluminum foil tightly around the neck of the
flask, covering the mouth, and use the pin or needle to
poke one tiny hole in the center of the foil. - Assemble the vapor density apparatus, as shown in
Figure 14-6. - Immerse the flask in the water bath, with the flask tilted
slightly to make the liquid more visible, and gradually
heat the flask until the liquid acetone boils. As the
acetone boils, acetone vapor displaces the air in
the flask.- Just as the final drop of acetone boils away, note the
temperature of the water bath (which, if you’ve been
heating it very gradually, is the same as the temperature
inside the flask). Record the temperature in kelvins on
line B of Table 14-5. - Immediately remove the flask from the water bath, and
run cold water over it. As the flask cools, the acetone
vapor condenses inside the flask.
Carefully dry the outside of the flask with a paper towel.
Reweigh the flask, aluminum foil, and condensed acetone,
and record that mass to 0.01 g on line C of Table 14-5.
Remove the aluminum foil from the flask, pour out as
much as possible of the condensed acetone, and then
replace the flask in the warm water bath. Allow the flask
to warm for a minute or two while you complete the
following calculations.
Calculate the mass of condensed acetone, and enter that
value on line D of Table 14-5.
Calculate the number of moles of condensed acetone,
and enter that value on line E of Table 14-5.
Remove the flask from the warm water bath and make
sure that no liquid remains in the flask. Use the graduated
cylinder to fill the flask with water, noting the total value of
water held by the flask when it is full to the brim. Record
that volume as accurately as possible on line F of
Table 14-5.
Use the barometer (or a local weather report) to
determine the atmospheric pressure. (See the note in
Lab 14.1 about atmospheric pressure versus barometric
pressure.) Record the atmospheric pressure on line G of
Table 14-5.
With the temperature, mass of acetone, volume, and
pressure known, you have all the information you need
to calculate the molar mass of acetone. Make that
calculation, and enter the value on line H of Table 14-5.