; Determination of the Vapor Pressure of Pure
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Determination of the Vapor Pressure of Pure


  • pg 1
									Determination of
    the Vapor
   Pressure of
•   Purposes and Demands
•   Principle
•   Apparatus and Reagent
•   Procedure
•   Data Records and Processing
•   Questions
•   Attentions
•   Demonstrated
   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
Clausius-Clapeyron equation.
 Determine the saturated vapor pressure
 of ethanol by the precision digital
 pressure gage under different
 temperature. Master the vacuum technique
 for determination.

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
vapor pressure.
   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
external pressure.
   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
                m 
                       H   气化

                      2.303  R

liquid, T stands for the absolute
temperature and R is the gas constant
of 8.314J·K-1·mol-1. C is the integral
constant .
   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
 • Thermometer
   Vacuum pump
 • Ethanol
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
  zero level.

• 3)Reset key: to restart-up.
• 4)Display    screen:   to   display
  pressure data.

 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
    5)   Fuse
 • 1.Assemble the equipments on the
   base   of    the  Sketch-maps of
   equipments above.

 • 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
  gas tank:
     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
the pressure.
   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:
 t    cmHg

   1. Proofread atmospheric pressure
 based on the equation
2、Vapour pressure (Pp) of ethanol at
difference temperatures:

      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
  too rapidly.
• 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


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