Transcript PC

Experiment 9. Liquid-Vapor Equilibrium in a Binary System
The vacuum manifold used in
this experiment is shown here. The
main stopcock isolates the system
from the pump, which is further
protected from the gaseous reagents
by a cold trap filled with liquid N2.
The MKS Baratron pressure gauge
converts pressures in Torr to voltages
(100 Torr = 1 V), which are read on a
voltmeter. The sample cell containing
liquid is attached at the middle
connector. The gas in equi-librium
with the liquid is captured
in the large bulb and is later condensed into a second sample cell attached to the
connector at the far right. (These are illustrated in more detail on the next page.)
The stopcock at left is used to bleed air into the system at the end of the
experiment.
2
The region of the cells and collection
bulb is shown here in larger scale. The
liquid sample cell is immersed in an ice
water bath for final equilibrium measurements. Make a rough determination of the
equilibrium P before opening the stopcock
to the collection bulb. After this P value is
known, open the stopcock to the bulb to
begin collecting vapor, and remove the ice
bath. This removal will offset the cooling of
the liquid due to the large amount of
evaporation and will speed up the collection
process. When P is near the equilibrium
value, reinstall the ice bath to achieve final
equilibrium. After this, close the sample
cell stopcock and move the gas to the other
cell by freezing it there with liquid N2.
3
Even with this procedure, it can take ~20 min to carry out all the operations
needed to obtain the equilibrium pressure and samples of liquid and gas present
at equilibrium for a single starting sample. Thus, teams should plan ahead so
that they are able to begin the work on the vacuum line as soon as possible
during the period. Start with pure acetone and pure chloroform, for which no
gas need be collected. Then go to the first mixture as soon as it has been
prepared. One or two partners should manage the vacuum line work while the
other(s) prepare mixtures and read refractive indices on the refractometer. In
this way you should be able to obtain pressure and composition data for the two
pure substances and at least six mixtures. Do more only if time permits.
(However, you should obtain calibration data for at least 10 mixtures and the
two pure components.)