Introduction
The equilibrium vapor pressure of a liquid is the pressure exerted by its vapor when it is in equilibrium with the liquid at a given temperature. The vapor pressure of a liquid always increases as the temperature increases. The normal boiling point of a liquid occurs when its vapor pressure is equal to one atmosphere. The vapor pressure of a liquid, p (in atmospheres), is related to the temperature at which it is measured, T (in Kelvin), by the Clausius-Clapeyron equation
where
C = a constant characteristic of the liquid under study
R = the gas constant (8.314 J/mol-K)
T = the temperature (in Kelvin)
D Hvap
= the molar heat of vaporization (J/mol)
ln p = the base e logarithm of the pressure
(atmospheres)
In this experiment you will measure the vapor pressure of an unknown liquid at the temperature of a cold water bath (between 0 and 5 degrees Celsius), at room temperature (around 15 to 20 degrees Celsius), and at the temperature of a warm water bath (between 30 and 35 degrees Celsius). Using the data collected you will be able to calculate the heat of vaporization of the liquid.
Determine the logarithmic value of the pressure. At each of the vapor pressure measurements, you will also have measured the temperature in degrees Celsius. Convert these to Kelvin by the addition of 273. Convert to the reciprocal, 1/T, and record the value as a decimal to at least three significant figures. Plot the three sets of points on graph paper, using the ln of the pressure as the y or vertical axis and the reciprocal of the temperature as the x or horizontal axis. Draw the best straight line that fits these points. The slope of this line is equal to the negative of the heat of vaporization divided by the gas constant.
Note that the equation you are using:
has the same form as the general equation for a straight line:
y = mx + b
where y = ln p , b (the y intercept) = C,
x = 1/T, and m (the slope) = 
You will also be asked to determine the normal boiling point of your unknown liquid. Use another form of the Clausius-Clapeyron Equation to do this.
or 
Use the ln p vs 1/T graph to find the ln p and 1/T values of a point on the straight line. Let these values be equal to ln p and 1/T . You have already determined D Hvap in this experiment. T2 will then be the normal boiling point occuring at p2 = 1 atm..
Equipment
three side-arm test tubes, 600 mL beaker, ring stand, utility clamp, three septums, thermometer, manometer (with its connecting tubing),three containers of the same unknown compound (at different temperatures; cold, room, and warm), three syringes (these come with each container of the unknown compound you will be using), wire gauze, stirring rod
Chemicals
students unknown liquids ( 0°
, 20° ,
40° )
instructor ( 0°
unknowns are in refrigerator, 20°
are out with lab setup , 40°
C are in the silver oven )
Disposal
Spill/Disposal F : Hazardous Waste Container
Provided
Procedure
1. Obtain three clean, dry, side-arm test tubes. Place a septum on each test tube. If you cannot find a dry side-arm test tube, then see your lab instructor. From the manometer remove the tubing that has the quick disconnect and is NOT connected to the manometer proper.Place the end of the tube that does NOT have the disconnect end on the side-arm of the test tube. Place a 600 mL beaker on a wire gauze, that is on an iron ring attached to a ring stand. Place the test tube in the 600 mL beaker. Clamp the tube to the ring stand. Add ice and water to the beaker so that the test tube is covered by the water-ice mixture up to the side-arm. Stir the water-ice bath several times to assure uniform temperature.
2. After 10 minutes (to assure thermal equilibrium has been reached) record the temperature of the water bath on the data sheet. Carefully connect the disconnect on the side-arm test tube to the disconnect on the manometer. This is done by pushing the disconnect parts together and then giving a half twist to assure a leak-proof seal.
Quickly inject 1.0 mL of the unknown liquid through the septum into the test tube using a hypodermic syringe. Remember to remove the syringe needle once the sample has been injected. The pressure in the system should increase as the liquid vaporizes. When the pressure reaches a maximum, record the difference in the height of the mercury in the manometer in units of mm. From this value you will have to subtract the constant value 11.4 mm. This is to correct for the change in the mercury level by the addition of 1.0 mL of a substance even if there were no vaporization of the liquid.
3. Repeat this procedure using a clean, dry side-arm test tube and using a water bath in the 15 to 20 degree range (around room temperature). Use hot or cold water to adjust the water temperature until it has a value equal to that of room temperature. Record the difference between the two levels of mercury (in mm) in the manometer and record the temperature of the water bath.
4. Repeat this procedure one more time using a clean, dry side-arm test tube and now using a water bath in the 35 to 40 degree temperature range. Again record the difference between the two levels of mercury in the manometer, and the temperature of the water bath.
Hand in the Preliminary Data Sheet as a group.
5. At home complete the calculations for the Data Sheet and, using a piece of the graph paper , plot the three sets of points .Draw the best straight line through these data points. Determine the slope of the line you have drawn and from the value of the slope determine the value of D Hvap.
6. Now using the value of D Hvap and one of the pressure and temperature points from the straight line you have on your graph, determine the normal boiling point of the liquid using the second form of the Clausius-Clapeyron Equation. Show all calculations.
Disposal
Any remaining liquid in the test tube must be disposed of into the labeled - "Equilibrium Vapor Pressure" Hazardous Waste container.