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

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```									Determination of
the Vapor
Pressure of
Contents
•   Purposes and Demands
•   Principle
•   Apparatus and Reagent
•   Procedure
•   Data Records and Processing
•   Questions
•   Attentions
•   Demonstrated
退出
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.
Principle
• 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
increases.
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
pressure
• 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
unit.

• 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
Procedure
• 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
• 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.
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 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，
35ºC，40ºC，45ºC，50ºC.
• 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:
H（in）
t    cmHg
t1
30℃
t2
40℃
t3
50℃
Processing

based on the equation
Pp（273.15/K）=P(T)[1-1.63×10-4(t-273.15K)]
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
lgP乙醇

30℃

40℃

50℃
Draw a plot of lgP versus 1/T and
figure out the slope m.
m=-ΔHv2.303R
lgP=lg101.325=

From the plot , 1/T＝
And boiling point T=
Questions
• 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
pressure?

• 4. What’s the use of the buffer gas
tank?
Attentions
• 1. The system must have the good
seal.

• 2. Determination should be done
after the liquid is boiled for 3-4
min.
• 3.Release a little pressure in
order to avoid the liquid boiling
too rapidly.
•
• 4.Regulate    the     temperature
accurately.
• 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
leakage.

• 7. Screw on the valves moderately.
• 8. Determine the data when the pressure