Experiment
Vapor Pressure of Liquids
10
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. Eventually, equilibrium is 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 Pressure Sensor and a Temperature Probe. The flask will be placed in water baths of
different temperatures 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.
Figure 1
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. Fill the 600 mL beakers halfway to prepare a water bath.
3. Prepare the Temperature Probe and Pressure Sensor for data collection.
a. Plug the Temperature Probe into Channel 1 of the computer interface.
b. Plug the Pressure Sensor into Channel 2 of the computer interface.
c. Obtain a rubber-stopper assembly with a piece of heavy-wall plastic tubing connected to
one of its two valves. Attach the connector at the free end of the plastic tubing to the open
stem of the Pressure Sensor with a clockwise turn. Leave its
two-way valve on the rubber stopper open (lined up with the
valve stem as shown in Figure 3) until Step 9.
d. Insert the rubber-stopper assembly into a 125-mL Erlenmeyer
flask. Important: Twist the stopper into the neck of the flask to
Figure 3
ensure a tight fit.
Chemistry with Computers
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Experiment 10
Figure 4
4. Prepare the computer for data collection by opening the Experiment 10 file from Chemistry
with Computers. The vertical axis will have pressure scaled from 90 to 135 kPa (100-130 are
labeled). The horizontal axis will have temperature scaled from 0 to 40°C.
5. The temperature and pressure readings should now be displayed in the Meter window. While
the two-way valve above the rubber stopper is still open, record the value for atmospheric
pressure in your data table.
6. Finish setting up the apparatus shown in Figure 4:
closed
open
a. Obtain a room-temperature water bath (20-25°C), you may need to
add warm water to get it there.
b. Hold the flask in the water bath, with the entire flask covered as
shown in Figure 4, if it isn’t add more water.
c. After 30 seconds, close the 2-way valve above the rubber stopper as
Figure 5
shown in Figure 5—do this by turning the white valve handle so it is
perpendicular with the valve stem itself.
d. Record this temperature and pressure as room temperature and pressure.
7. Take your syringe to the fume hood. Draw 3 mL of the methanol up into the syringe. With
the two-way valve still closed, screw the syringe onto the valve, as shown in Figure 4.
8. Introduce the methanol into the Erlenmeyer flask.
a. Open the 2-way valve above the rubber stopper—do this by turning the white valve handle
so it is aligned with the valve stem (see Figure 5).
b. Squirt the methanol into the flask by pushing in the plunger of the syringe.
c. Quickly return the plunger of the syringe back to the 3-mL mark of the syringe, then close
the valve by turning the white valve handle so it is perpendicular with the valve stem.
9. Collect pressure and temperature data. Click
Collect
.
10. Warm the water bath to 30-35°C:
a. Pour some of your water out and add hot water until it gets to the correct temperature.
b. Place the Erlenmeyer flask assembly and the temperature probe into the 30-35°C water
bath. Make sure the entire flask is covered with water.
c. Stir the water continuously.
d. After one minute, click Keep .
e. Record these values as trial1.
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Chemistry with Computers
Vapor Pressure of Liquids
11. Pour out some of your water and add ice. Make sure the entire flask is covered with water.
Continually stir your water. When you reach a temperature 20-22°C water bath click Keep .
Record these values as trial 2.
12. Continue to stir, you may need to add more ice and pour out some water. Make sure the entire
flask is covered with water. Click Keep when you obtain a temperature of 11-12o C, record
these values as trial 3. Click Keep when you obtain a temperature of 5-6oC. Record these
values as trial 4.
13. Click
table.
Stop
to end data collection. Record the pressure and temperature values in your data
14. Open the side valve of the Pressure Sensor so the Erlenmeyer flask is open to the atmosphere.
Remove the stopper assembly from the flask and dispose of the methanol in the beaker
labeled methanol in the fume hood not the waste container.
15. Clean and dry your 125-mL Erlenmeyer flask. Draw air in and out of the syringe enough
times that you are certain that all of the methanol has evaporated from it.
16. Repeat Steps 6-12 using ethanol in the room temperature water bath. When you click collect,
hit the option Store Latest Run.
17. Open the 2-way valve of the Pressure Sensor. Remove the stopper assembly from the flask
and dispose of the ethanol in the beaker labeled ethanol in the fume hood not the waste
container.
PROCESSING THE DATA
1. Convert each of the Celsius temperatures to Kelvin (K). Write the answer in the space
provided.
2. To obtain the vapor pressure of methanol and ethanol, the air pressure must be subtracted
from each of the measured pressure values. However, even if no methanol was present, the
pressure in the flask would have changed due to change in temperature. Therefore, you must
convert the atmospheric pressure to a corrected air pressure at the temperature of the water
bath in all trials. To do this, use the gas-law equation (use the Kelvin temperatures):
P2
P1
=
T2
T1
where P1 and T1 are the atmospheric pressure and the temperature of the room. T2 is the
temperature of the water bath in Trials 1, 2, 3, or 4. Solve for P2, and record this value as the
corrected air pressure.
3. Obtain the vapor pressure by subtracting the corrected air pressure from the measured
pressure in Trials 1-4.
4. On Logger Pro, click Data…New Manual Column… Change the name to vapor pressure.
Change the units to kPa. Click OK. Type your values for vapor pressure for both trials in
these new columns, you can move the cursor right or left to find the new columns on logger
pro.
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Experiment 10
5.
On your graph right click on the word pressure on the horizontal axis, and change it to vapor
pressure. Label the ethanol and methanol vapor pressure series. To do this click Insert
…Text Annotation. Change the scale of your graph so your data points are visible. Just click
on the lowest and highest number on the horizontal axis, and type in a new value. Print the
graph of vapor pressure vs. temperature (°C).
6. How would you describe the relationship between vapor pressure and temperature, as
represented in the graph you made in the previous step? Explain this relationship using the
concept of kinetic energy of molecules.
6. Which liquid, methanol or ethanol, had the larger vapor pressure value? Explain your answer.
Compare the intermolecular forces in these two liquids.
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Chemistry with Computers
Vapor Pressure of Liquids
DATA AND CALCULATIONS
Room pressure and temperature
________ oC
_______ kPa
Substance
Methanol
Trial
1
2
3
4
Temperature
(°C)
°C
°C
°C
°C
Temperature
(K)
K
K
K
K
kPa
kPa
kPa
kPa
Measured
pressure
Air pressure
Corrected
Corrected
Corrected
Corrected
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
Vapor
pressure
Room pressure and temperature
________ oC
_______ kPa
Substance
Ethanol
Trial
1
2
3
4
Temperature
(°C)
°C
°C
°C
°C
Temperature
(K)
K
K
K
K
kPa
kPa
kPa
kPa
Measured
pressure
Air pressure
Corrected
Corrected
Corrected
corrected
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
Vapor
pressure
Chemistry with Computers
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