Determination of the Vapor Pressure of Pure Liquid

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Determination of the Vapor Pressure of Pure
Liquid
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Contents

• Purposes and Demands
• Principle
• Apparatus and Reagent
• Procedure
• Data Records and Processing
• Questions
• Attentions
• Demonstrated
• ??

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• 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.
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• ? 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.

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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.

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• 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.

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• 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(?Hv).

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• 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 8.314JK-1mol-1. C is the
integral constant .

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• A plot of lgP versus 1/T will be a straight
  line. ?Hv can be determined by the line slope m.

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Apparatus and Reagent

• The measuring device of the Vapor pressure
• thermostatic bath
• Precision digital pressure gage
• buffer gas tank
• Thermometer
  Vacuum pump
• Ethanol

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Sketch-maps of Equipments
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DF-AF precision digital pressure gageThe front
panel of the device
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• 1)Units key to choose a measuring unit.
• 2)Zero key to deduct pressure at zero level.
• 3)Reset key to restart-up.

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• 4)Display screen to display pressure data.
• 5)Indicator light to display the unit of
  the pressure.

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• 1)   Electrical outlet
• 2)   Facet contacted with host computer
• 3)   Facet contacted with pressure canal
• 4)   Pressure-regulating
• 5) Fuse

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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.

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• 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.

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• 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.
  

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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Data Records and Processing

• 1) Records
• Room temperature
• Atmospheric pressure

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Processing
1.     Proofread atmospheric pressure based
on the equation Pp(273.15/K)P(T)1-1.6310-4(t-27
3.15K)
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2?Vapour pressure (Pp) of ethanol at difference
temperatures
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Draw a plot of lgP versus 1/T and figure out the
slope m. m-?Hv2.303R lgPlg101.325 From the
plot , 1/T And boiling point T
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Questions

• 1. What condition is the Clausius-Clapeyron
  equation based on?
• 2. Whats the result if air in the U tube has not
  extracted completely?

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• 3. Can the method be used to determine the
  solutions vapor pressure?
• 4. Whats the use of the buffer gas tank?

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Attentions

• 1. The system must have the good seal.
• 2. Determination should be done after the liquid
  is boiled for 3-4 min.

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• 3.Release a little pressure in order to avoid the
  liquid boiling too rapidly.
• 4.Regulate the temperature accurately.

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• 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.

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• 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 determination.
•  

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