• Purposes and Demands
• Apparatus and Reagent
• Data Records and Processing
Purposes and Demands
Understand the vapor pressure of pure
liquid and gas-liquid equilibrium.
Acquaint with the relationship of the
vapor pressure and temperature by the
Determine the saturated vapor pressure
of ethanol by the precision digital
pressure gage under different
temperature. Master the vacuum technique
Determine the molar heat of
vaporization and boiling point by
the method of illustration.
• The molecules of a pure liquid
filled in an equilibrium tube will
transform into the phase of gas and
liquid when the pressure of the
vapor in the tube reaches a
definite value. This pressure is
called the vapor pressure of the
liquid at a given temperature.
Temperature is the main factor of the
The vapor pressure increases up to
the critical point as the temperature
The vaporization with the bubbles
formation will occurs in the interior of
the liquid as well as at the surface
when the liquid may be heated up to a
temperature and its vapor pressure is
equal to the external pressure.
It is quite the boiling point of
the liquid at the specified
The heat consumed to vaporize 1
molar liquid is called the
gasification latent heat(ΔHv).
Clausius-Clapeyron equation is known as
Where P is the vapor pressure of
liquid, T stands for the absolute
temperature and R is the gas constant
of 8.314J·K－1·mol－1. C is the integral
A plot of lgP versus 1/T will be
a straight line. ΔHv can be
determined by the line slope m.
Apparatus and Reagent
• The measuring device of the Vapor
• thermostatic bath
• Precision digital pressure gage
• buffer gas tank
Sketch-maps of Equipments
DF-AF precision digital pressure gage
The front panel of the device
• 1)Units key: to choose a measuring
• 2)Zero key: to deduct pressure at
• 3)Reset key: to restart-up.
• 4)Display screen: to display
5)Indicator light: to display the
unit of the pressure.
• 1) Electrical outlet
• 2) Facet contacted with host computer
• 3) Facet contacted with pressure canal
• 4) Pressure-regulating
• 1.Assemble the equipments on the
base of the Sketch-maps of
• 2.Fill with the ethanol to the two-
third volume of the tube . Startup
the vacuum pump.
• 1. Check the seal of the buffer gas tank :
• 1) Test the gas leakage of the buffer
Open the valve 2 and screw on the
valve 1, then startup the vacuum pump.
When the pressure is up to the range of
100kpa-200kpa, stop pumping and close
the air inlet.
The system has good seal if the
pressure falls down at less than
0.01kpa/s, or else we must workout
the problem before next procedure.
• 2) Check seal of the gas trimmer:
Open the valve 1 when the pump and
air inlet and the valve 2 are close.
Regulate the pressure which is less
than the 1/2 pressure of tank by
the valve 1.
It shows that the trimmer has the
good seal if the pressure falls
down at less than +0.01kPa/4s, or
else the question should be
resolved before your experiment.
• 3) Check the seal of the determined part:
The pressure of the trimmer is
equal to that of the gas tank when the
valve 2 is open and the valve 1 is close.
Then shut the valve 2 and and open the
valve 1, and release pressure moderately.
Observe the pressure reading.
It shows that it has the good seal
if the pressure change is less than
or equal to 0.01kpa/4s. If not, we
must find out and resolve the
problem before next step.
• 4. Determination step
Set up the temperature of the
water bath at 30℃. Startup the
vacuum pump and the condenser tube.
Open the valve 2, and carefully let
the air bubble steadily through
the U tube. Then close valve 2 and
open valve 1.
After 3-4 min, adjust the two
valves until the two levels of the
liquid in the U tube are the same.
Read and record the temperature and
Repeat the same experiment ,in
which the pressure difference is
less than the range of 67pa.
• On the base of the above procedure,
determine the vapor pressure
respectively at the temperature of 30ºC，
• Release pressure in the gas tank and
disassemble the equipments. Stop the
condenser tube and cut down the current.
Data Records and Processing
• 1) Records
• Room temperature:
• Atmospheric pressure:
1. Proofread atmospheric pressure
based on the equation
2、Vapour pressure (Pp) of ethanol at
T/K 1/T Δh0 P乙醇
t℃ 273+t℃/K T-1×10-3/K-1 MPa MPa
Draw a plot of lgP versus 1/T and
figure out the slope m.
From the plot , 1/T＝
And boiling point T=
• 1. What condition is the Clausius-
Clapeyron equation based on?
• 2. What’s the result if air in the
U tube has not extracted completely?
• 3. Can the method be used to
determine the solution’s vapor
• 4. What’s the use of the buffer gas
• 1. The system must have the good
• 2. Determination should be done
after the liquid is boiled for 3-4
• 3.Release a little pressure in
order to avoid the liquid boiling
• 4.Regulate the temperature
• 5. The U tube must be placed under
the level of the water bath.
• 6. Examine carefully the two valves
1 and 2, and see if they have
• 7. Screw on the valves moderately.
• 8. Determine the data when the pressure
reading is steady.
• 9. If air is pumped into the U tube, you
need vacuumize it again before