Exp 10: Vapor Pressure of Liquids
In this experiment, you will investigate the relationship between the vapor pressure of a liquid and its
temperature. When a liquid is added to the Erlenmeyer flask shown in Figure 1, it will evaporate into the air
above it in the flask. If the stopper is left open, eventually all of the air will be displaced by methanol vapor.
Once sealed, equilibrium will be reached between the rate of evaporation and the rate of condensation. At
this point, the vapor pressure of the liquid is equal to the partial pressure of its vapor in the flask. Pressure
and temperature data will be collected using a Gas Pressure Sensor and a Temperature Probe. The flask will
be placed in a water bath heated to just above the boiling point of the liquid being investigated and then
allowed to slowly cool to determine the effect of temperature on vapor pressure. You will also compare the
vapor pressure of two different liquids, ethanol and methanol, at the same temperature.
OBJECTIVES
In this experiment, you will


Investigate the relationship between the vapor pressure of a liquid and its temperature.
Compare the vapor pressure of two different liquids at the same temperature.
Figure 1a
Figure 1b
Figure 1c
MATERIALS
computer
Vernier computer interface
Logger Pro
Vernier Gas Pressure Sensor
Vernier Temperature Probe
rubber-stopper assembly
plastic tubing with two connectors
two 125 mL flasks
Methanol or ethanol
Ring stand and clamp
600 mL beaker
PRELAB ASSIGNMENT
1. Why is waiting to see liquid at the open fitting important?
2. Read 13.12 – 13.14 in your textbook.
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PROCEDURE
1. Obtain and wear goggles! CAUTION: The alcohols used in this experiment are flammable and
poisonous. Avoid inhaling their vapors. Avoid contacting them with your skin or clothing. Be sure there
are no open flames in the lab during this experiment. Notify your teacher immediately if an accident
occurs.
2. Use a 600 mL beaker to prepare a water bath. Start heating the water.
3. Prepare the Flask, Temperature Probe, and Gas Pressure Sensor for data collection.
a. Attach the ~20cm length of clear, colorless tubing to the Gas Pressure Sensor. Do not over-tighten.
Turning by clamping the nylon Luer-Lok between extended index finger and thumb will limit the
amount of force you can apply.
b. Plug the Gas Pressure Sensor into CH1 and the Temperature Probe into CH2 of the computer
interface. The Gas Pressure Sensor should be hung such that it is well above the flask and the hose is
mostly vertical. See Figure 1c. Note in the same figure how the Gas Pressure Sensor and Temperature
Probe cords are routed so as to not come into contact with the hot plate!
c. Obtain a rubber-stopper assembly with a Luer-Lok [http://en.wikipedia.org/wiki/Luer-Lok] insert in
place. Do not attach the tube or Pressure Sensor. Pour ~10 mL of the liquid to be investigated into the
flask. Insert the rubber-stopper assembly into the flask. Important: Twist the stopper into the neck of
the flask to ensure a tight fit.
4. Prepare the computer for data collection. You will be using the default sensor setup, but will need to
modify the data collection setup so that data is collected once per second for 3000 seconds.
5. The temperature and pressure readings should now be displayed in the meter. While the Luer-Lok is
open, record the value for atmospheric pressure and room temperature in your data table (round to the
nearest 0.1 kPa and 0.1 oC).
6. Finish setting up the apparatus shown in Figures 1b and 1c:
a. Make sure the stir bar is spinning.
b. Place the Temperature Probe in the water bath. You do not want the water bath to exceed 15oC above
the boiling point of the liquid in the flask.
c. Hold the flask in the water bath, with the entire flask covered as shown in Figure 1b and tighten the
clamp to the neck of the flask.
d. The liquid in the flask should start to boil (more of a “simmer”, not a “rolling boil”). After two
minutes of boiling note what is happening at the Luer-Lok fitting on the stopper. You should see the
presence of liquid. If not, do not proceed.
e. Click
after the liquid has been boiling 2 minutes. There should still be liquid in the flask! Turn
the hot plate off. Do not turn off the stirring function.
f. Attach the GPS hose to the Luer-Lok adapter on the stopper. Note what happens inside the hose in the
first few minutes.
7. Periodically check to see that your data collection is going as planned, but use the time to work on
something.
8. Gently loosen and remove the Gas Pressure Sensor so the flask is open to the atmosphere. Remove the
stopper assembly from the flask and dispose of the methanol as directed by your teacher.
9. Place copies of the P vs. t and T vs. t plots in your lab manual.
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PROCESSING THE DATA
When asked to “compare to the accepted value” you are expected to calculate percent error and provide
plausible, specific, non-random explanations for the error.
1. Create and copy into your lab manual a plot of P vs. T for the/each liquid. Mark on each plot where the
pressure inside the flask was equal to the ambient pressure. Which liquid had the larger vapor pressure
value at atmospheric pressure? Show this on your plots. Explain why this is, taking into account any and
all factors that affect vapor pressure.
2. Use the Examine function to determine the time at which the vapor pressure was equal to the atmospheric
pressure. Using this time, determine the temperature of the liquid-gas mixture in the flask. What is this
temperature called? Compare to the accepted value.
Repeat, but instead of atmospheric pressure use standard pressure.
3. The Clausius-Clapeyron equation describes the relationship between vapor pressure and absolute
temperature:
ln P  H vap / RT  B
where ln P is the natural logarithm of the vapor pressure, Hvap is the heat of vaporization, T is the
absolute temperature, B is a positive constant, and R is the ideal gas constant. If this equation is
rearranged in slope-intercept form (y = mx + b):
 H vap 1
ln P 
 B
R
T
the slope, m, should be equal to –Hvap / R. If a plot of ln P vs. 1/T is made, the heat of vaporization can
be determined from the slope of the curve. You will plot the appropriate graph using LoggerPro.
You will need to perform several
mathematical operations on the data. As you
have collected thousands of data points,
having the computer do this seems logical.
a. The first step is to add a column and
populate its cells with the temperature
measured in Kelvin. Choose Data / New
Calculated Column and set up the column as
shown to the right.
b. Create a New Calculated Column that
shows the reciprocal of the temperature
measured in Kelvin: 1 / T(K).
c. Create a New Calculated Column that
shows the natural logarithm of Pressure:
ln(P). Note that the log function removes units.
4. For the liquid, go to Insert / Graph to create a plot of ln(P) vs. 1 / T(K). Not all of the data is to be used.
Recall what was happening when you first connected the tube to the stopper. Determine the equation of
the best-fit line for each. [If the plots are not linear then either you did something wrong or the data is not
good. Check your work.] Copy the graphs, with the curve-fit dialogue windows shown, into your lab
manual.
Determine the ranges of temperature and pressure that correspond to your plot being linear. Mark these
regions on your P vs. t and T vs. t plots using brackets ( […] ). Are the magnitudes of these ranges large
enough to expect a reasonable result for heat of vaporization?
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5. Calculate the heat of vaporization for the liquid. Compare to the accepted value.
6. A student carries out the lab but fails to follow one specific instruction: Instead of waiting to attach the
Gas Pressure Sensor, the student connects the Gas Pressure Sensor immediately after adding the liquid
and inserting the stopper. How will this failure to follow instructions affect the student’s results? Explain
all effects, using words and/or equations.
7. How might this lab be improved? Offer specific, doable suggestions and justify the reason for each.
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