### 10 Vapor Pressure of Liquids Computer

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Vapor Pressure of Liquids
10
py
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 Gas 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.
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OBJECTIVES
In this experiment, you will
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 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 1
MATERIALS
Ev
computer
Vernier computer interface
Logger Pro
Vernier Gas Pressure Sensor
Vernier Temperature Probe
rubber-stopper assembly
plastic tubing with two connectors
Chemistry with Vernier
20 mL syringe
methanol
ethanol
ice
four 1 liter beakers
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Computer 10
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 1 liter beakers to prepare four water baths, one in each of the following temperature
ranges: 0 to 5°C, 10 to 15°C, 20 to 25°C (use room temperature water), and 30 to 35°C. For
each water bath, mix varying amounts of warm water, cool water, and ice to obtain a volume
of 800 mL in a 1 L beaker. To save time and beakers, several lab groups can use the same set
of water baths.
3. Prepare the Temperature Probe and Gas Pressure Sensor for data collection.
a. Plug the Gas Pressure Sensor into CH1 and the Temperature Probe
into CH2 of the computer interface.
b. 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 Gas Pressure Sensor with a clockwise turn. Leave its two-way
Figure 2
valve on the rubber stopper open (lined up with the valve stem as
shown in Figure 2) until Step 9.
c. Insert the rubber-stopper assembly into a 125 mL Erlenmeyer flask. Important: Twist the
stopper into the neck of the flask to ensure a tight fit.
Figure 3
4. Prepare the computer for data collection by opening the file “10 Vapor Pressure” from the
Chemistry with Vernier folder of Logger Pro.
5. The temperature and pressure readings should now be displayed in the meter. While the twoway valve above the rubber stopper is still open, record the value for atmospheric pressure in
your data table (round to the nearest 0.1 kPa).
6. Finish setting up the apparatus shown in Figure 3:
a. Obtain a room-temperature water bath (20–25°C).
b. Place the Temperature Probe in the water bath.
c. Hold the flask in the water bath, with the entire flask covered as
shown in Figure 3.
d. After 30 seconds, close the 2-way valve above the rubber stopper as
shown in Figure 4—do this by turning the white valve handle so it is
perpendicular with the valve stem itself.
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open
closed
Figure 4
Chemistry with Vernier
Vapor Pressure of Liquids
7. Obtain the methanol container and the syringe. Draw 3 mL of the methanol up into the
syringe. With the two-way valve still closed, screw the syringe onto the two-way valve, as
shown in Figure 3. DANGER: Methanol, CH3OH: Keep away from heat, sparks, open
flames, and hot surfaces—highly flammable liquid and vapor. Toxic if swallowed, in contact
with skin, or if inhaled. Do not eat or drink when using this product. Do not breath mist,
vapors, or spray. Causes skin and serious eye irritation. Causes damage to organs.
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 4).
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 2-way valve by turning the white valve handle so it is perpendicular with the valve
stem.
d. Remove the syringe from the 2-way valve with a counter-clockwise turn.
9. To monitor and collect pressure and temperature data:
a. Click
.
b. When the pressure and temperature readings displayed in the meter stabilize, equilibrium
between methanol liquid and vapor has been established. Click
. The first pressuretemperature data pair is now stored.
10. To collect another data pair using the 30–35°C water bath:
a. Place the Erlenmeyer flask assembly and the temperature probe into the 30–35°C water
bath. Make sure the entire flask is covered.
b. When the pressure and temperature readings displayed on the computer monitor stabilize,
click
. The second data pair has now been stored.
11. For Trial 3, repeat the procedure in Step 10, using the 10–15°C water bath. Then, repeat the
same procedure for Trial 4, using the 0–5°C water bath.
12. Click
to end data collection. Record the pressure and temperature values in your data
table, or, if directed by your instructor, print a copy of the table.
13. Gently loosen and remove the Gas Pressure Sensor so the Erlenmeyer flask is open to the
atmosphere. Remove the stopper assembly from the flask and dispose of the methanol as
14. Obtain another clean, dry 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 6–8 to do one trial only using ethanol in the room temperature water bath.
When the pressure stabilizes, record the measured pressure of ethanol displayed in the meter
in your data table. DANGER: Denatured ethanol, CH3CH2OH: Highly flammable liquid and
vapor. Keep away from heat, sparks, open flames, and hot surfaces. Do not eat or drink when
using this product—harmful if swallowed. Causes skin and serious eye irritation. May cause
respiratory irritation. Avoid breathing mist, vapors, or spray. Causes damage to organs.
17. Gently loosen and remove the stopper assembly from the flask and dispose of the ethanol as
Chemistry with Vernier
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Computer 10
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, for Trials 2–4, even if no methanol was
present, the pressure in the flask would have increased due to a higher temperature, or
decreased due to a lower temperature (remember those gas laws?). Therefore, you must
convert the atmospheric pressure at the temperature of the first water bath to a corrected air
pressure at the temperature of the water bath in Trial 2, 3, or 4. 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 Trial 1 (room
temperature) water bath. T2 is the temperature of the water bath in Trial 2, 3, or 4. Solve for
P2, and record this value as the corrected air pressure for Trials 2, 3, and 4. For Trial 1 of
methanol and Trial 1 of ethanol, it is not necessary to make a correction; for these two trials,
simply record the atmospheric pressure value in the blank designated for air pressure.
3. Obtain the vapor pressure by subtracting the corrected air pressure from the measured
pressure in Trials 2-4. Subtract the uncorrected air pressure in Trial 1 of methanol (and
Trial 1 of ethanol) from the measured pressure.
4. Plot a graph of vapor pressure vs. temperature (°C) for the four data pairs you collected for
methanol. Temperature is the independent variable and vapor pressure is the dependent
variable. As directed by your teacher, plot the graph manually, or use Logger Pro. Note: Be
sure to plot the vapor pressure, not the measured pressure.
5. 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 at room temperature?
Explain your answer. Take into account various intermolecular forces in these two liquids.
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Chemistry with Vernier
Vapor Pressure of Liquids
DATA AND CALCULATIONS
Atmospheric pressure
_______ kPa
Substance
Methanol
Trial
1
2
Ethanol
3
4
1
Temperature
(°C)
°C
°C
°C
°C
°C
Temperature
(K)
K
K
K
K
K
kPa
kPa
kPa
kPa
kPa
Measured
pressure
Air pressure
no correction
corrected
corrected
corrected
no correction
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
Vapor
pressure
EXTENSION
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, and B is a positive constant. If this equation is rearranged in slopeintercept 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. Plot the graph using Logger Pro:
1. Go to Page 2 of the experiment file by clicking on the Next Page button,
.
2. In the table, enter the four vapor pressure-temperature data pairs. To do this:
a. Click on the first cell in the Temperature (K) data column in the table. Type in
temperature value (K) for the first data pair, and press the ENTER key.
b. The cursor will now be in the Vapor Pressure (kPa) data column—type in its value and
press ENTER.
c. Continue in this manner to enter the last three data pairs values.
d. If necessary, click on the Autoscale button, , to automatically rescale the data points.
Chemistry with Vernier
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Computer 10
3. Create a column 1/Temperature (in units of 1/K). To do this:
a. Choose New Calculated Column from the Data menu.
b. Enter “1/Temperature” as the Name, “1/Temp” as the Short Name, and “1/K” as the Unit.
c. Enter the correct formula for the column (1/Temperature) into the Equation edit box. To
do this, type in “1” and “/”. Then select “Temperature Kelvin” from the Variables list. In
the Equation edit box, you should now see displayed: 1/“Temperature”.
d. Click
.
4. Create a column ln Vapor Pressure. To do this:
a. Choose New Calculated Column from the Data menu.
b. Enter “ln Vapor Pressure” as the Name, “ln V Press” as the Short Name. You do not need
to enter a unit.
c. Enter the correct formula for the column into the Equation edit box. Choose “ln” from the
Function list. Then select “Vapor Pressure” from the Variables list. In the Equation edit
box, you should now see displayed: ln(“Vapor Pressure”). Click
.
d. Click on the vertical axis label and choose ln Vapor Pressure.
e. Click on the horizontal axis label and choose 1/Temperature.
f. Autoscale the graph by clicking on the Autoscale button, , on the toolbar.
g. Click the Linear Fit button, .
5. From the Regression Statistics option, find the slope, m, of the regression line.
6. Use the slope value to calculate the heat of vaporization for methanol (m = –Hvap / R).
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Chemistry with Vernier
Vernier Lab Safety Instructions Disclaimer
THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB.
This copy does not include:

Safety information

Essential instructor background information

Directions for preparing solutions

Important tips for successfully doing these labs
The complete Chemistry with Vernier lab manual includes 36 labs and essential teacher
information. The full lab book is available for purchase at http://www.vernier.com/cwv
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