Determination of the Vapor Pressure of Pure Liquid

Contents Purposes and DemandsPurposes and Demands Principle Apparatus and ReagentApparatus and Reagent Procedure Data Records and ProcessingData 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 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 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 30 º C ， 35 º C ， 40 º C ， 45 º C ， 50 º C. 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 H （ in ） cmHg t 1 30 ℃ t 2 40 ℃ t 3 50 ℃

Processing 1. Proofread atmospheric pressure based on the equation P p （ /K ） =P(T)[1-1.63×10 -4 (t K)]

2 、 Vapour pressure (P p ) of ethanol at difference temperatures: t℃t℃ T/K 273+t ℃ /K 1/T T -1 ×10 -3 /K -1 Δh 0 MPa P 乙醇 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=lg = 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 reading is steady. 9. If air is pumped into the U tube, you need vacuumize it again before determination.