# Vapor Pressure of Liquids

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1 Vapor Pressure of Liquids Experiment 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 MATERIALS TI-83 Plus or TI-84 Plus graphing calculator EasyData application data-collection interface Temperature Probe Vernier Gas Pressure Sensor four 1 liter beakers 20 ml syringe rubber-stopper assembly plastic tubing with two connectors two 125 ml Erlenmeyer flasks methanol ethanol ice 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. Chemistry with Calculators 2006 Vernier Software & Technology 10-1

2 Experiment Prepare the Temperature Probe and Pressure Sensor for data collection. a. Turn on the calculator and connect it to the data-collection interface. b. Connect the Temperature Probe to Channel 1 of the interface. c. Connect the Pressure Sensor to Channel 2 of the interface. d. 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 Figure 2 shown in Figure 2) until Step 6. e. 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. Set up EasyData for data collection. a. Start the EasyData application, if it is not already running. b. Select from the Main screen, and then select New to reset the application. c. Select from the Main screen, and then select Selected Events. 5. The temperature and pressure readings should now be displayed on the calculator screen. While the two-way valve above the rubber stopper is still open, record the value for atmospheric pressure in your data table (round to the nearest 0.1 ). 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 two-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. open closed Figure Chemistry with Calculators

4 Experiment 10 PROCESSING THE DATA 1. Convert each of the Celsius temperatures to Kelvin (K). Record the answers. 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 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): P 2 P = 1 T2 T1 where P 1 and T 1 are the atmospheric pressure and the temperature of the Trial 1 (room temperature) water bath. T 2 is the temperature of the water bath in Trial 2, 3, or 4. Solve for P 2, and record this value as the 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 air pressure from the measured pressure in Trials 2 4. Subtract the un 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 the dependent variable. As directed by your instructor, plot the graph manually or use Logger Pro software. 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 Chemistry with Calculators

5 Vapor Pressure of Liquids DATA AND CALCULATIONS Atmospheric pressure Substance Methanol Ethanol Trial Temperature ( C) Temperature (K) Measured pressure C C C C C K K K K K Air pressure no correction no correction 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, H vap 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): ln P = H vap / R 1 T + B the slope, m, should be equal to H vap / 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 on the calculator: 1. Enter the temperature ( C) and vapor pressure values into your calculator: a. To view the data lists, press to display the EDIT menu, and then press to select Edit. b. Clear L1 and L 2 by moving the cursor to the L1 or L 2 headings and pressing. c. Enter the four Celsius temperature values in L1 and the four corresponding vapor pressure values in L 2. Important: Enter the values in order of increasing temperature. d. To create a list of reciprocal of Kelvin temperature values (in L 3), move the cursor to the L3 column heading, then press [L1] e. To create a list of natural log (ln) of vapor pressure values (in L 2), move the cursor to the L2 column heading, then press [L 2]. Proceed to Step 2. Chemistry with Calculators 10-5

6 Experiment Follow this procedure to calculate regression statistics and to plot a best-fit regression line on your graph of ln pressure vs. reciprocal of Kelvin temperature: a. Start EasyData. b. Select to view the Graph screen. c. Select and then select L2 vs L3 to view a graph of ln vapor pressure vs. 1/Kelvin temperature. d. Select, then select Linear Fit. The linear-regression statistics for these two lists are displayed for the equation in the form: y = ax + b where x is 1 / Kelvin temperature, y is ln vapor pressure, a is the slope, and b is the y- intercept. Record the value of the slope, a, to use in calculating the heat of vaporization in Step 3. e. To display the linear-regression curve on the graph of ln vapor pressure vs. 1/Kelvin temperature (K), select. Examine your graph to see if the relationship between ln pressure vs. 1/ Kelvin temperature is linear. f. (optional) Print a copy of the graph. 3. Use the slope value you recorded in Step 2 to calculate the heat of vaporization for methanol (m = a = H vap / R) Chemistry with Calculators

7 Experiment 10 TEACHER INFORMATION Vapor Pressure of Liquids 1. This activity can be performed with calculators from the TI-83 Plus or TI-84 Plus families and a LabPro or CBL 2. It can not be performed with Easy products because the Temperature Probe and Gas Pressure Sensor must collect data simultaneously. 2. Have a good supply of room-temperature tap water available for students to use for the Trial 1 water bath. Room-temperature water helps prevent the water-bath temperature from changing between Trial 1 of methanol and Trial 1 of ethanol. This improves the validity of comparing their vapor pressures. 3. Emphasize to your students the importance of providing an airtight fit with all plastic-tubing connections and when closing valves or twisting the stopper into the Erlenmeyer flask. 4. All of the pressure valves, tubing, and connectors used in this experiment are included with Vernier Pressure Sensors shipped after February 15, These accessories are also helpful when performing vapor pressure experiments such as Experiment 7 in this manual, Pressure- Temperature Relationship in Gases If you purchased your pressure sensor at an earlier date, Vernier has a Pressure Sensor Accessories Kit (PS-ACC) that includes all of the parts shown here for doing pressuretemperature experiments (except the Erlenmeyer flask). Using this kit allows for easy assembly of a completely airtight system. The kit includes the following parts: two ribbed, tapered valve connectors inserted into a No. 5 rubber stopper one ribbed, tapered valve connectors inserted into a No. 1 rubber stopper two Luer-lock connectors connected to either end of a piece of plastic tubing one two-way valve one 20 ml syringe two tubing clamps for transpiration experiments 5. Connect the piece of plastic tubing to the shorter valve connector on the rubber stopper prior to the start of class (as shown here). Threaded Luer-lock connectors are attached to both ends of the plastic tubing. Connect one of these to the valve stem with a 1/2 clockwise turn. 6. Even though this procedure requires students to calculate changes in total gas pressure due to changes in temperature, we felt it was less complicated than connecting the container to a vacuum pump or aspirator. Your students may be surprised when the pressure actually drops below the original air pressure at low temperatures. 7. The Vernier temperature calibrations that are stored in the EasyData application work well for this experiment. The stored pressure calibrations for the Gas Pressure Sensor also work well for this experiment. 8. Discuss with your students the relationship between vapor pressure and the rates of evaporation and condensation. It is important for students to relate the resulting equilibrium to the stabilizing of the pressure they observe in the experiment. Remind them to look for the presence of some remaining liquid once equilibrium is reached, since there can be no equilibrium without some liquid being present. There is little chance of all of the liquid Chemistry with Calculators 2006 Vernier Software & Technology 10-1 T

8 Experiment 10 evaporating; approximately 1 2 ml of liquid will be delivered, but only about 0.05 ml will actually evaporate at the highest temperature. 9. HAZARD ALERTS: Methanol: Flammable; dangerous fire risk; toxic by ingestion (ingestion may cause blindness). Hazard Code: B Hazardous. Ethanol: Dangerous fire risk; flammable; addition of denaturant makes the product poisonous it cannot be made non-poisonous; store in a dedicated flammables cabinet or safety cans. If a flammables cabinet or safety cans are not available, store in a Flinn Saf-Stor Can. Hazard Code: C Somewhat hazardous. The hazard information reference is: Flinn Scientific, Inc., Chemical & Biological Catalog Reference Manual, 2000, P.O. Box 219, Batavia, IL See Appendix F of this book, Chemistry with Calculators, for more information. ANSWERS TO QUESTIONS Answers have been removed from the online versions of Vernier curriculum material in order to prevent inappropriate student use. Graphs and data tables have also been obscured. Full answers and sample data are available in the print versions of these labs. SAMPLE DATA AND CALCULATIONS Atmospheric pressure Substance Methanol Ethanol Trial Temperature ( C) xxxx xxxx xxxx xxxx xxxx Temperature (K) xxxx xxxx xxxx xxxx xxxx Measured pressure xxxx xxxx xxxx xxxx xxxx Air pressure no correction no correction Vapor pressure xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx 10-2 T Chemistry with Calculators

9 Teacher Information Vapor Pressure of Liquids EXTENSION Vapor Pressure (Methanol) vs. Celsius Temperature Using the slope obtained from the graph of ln vapor pressure vs. 1/temperature (K) for methanol (see the sample graph and regression statistics on the next page), the heat of vaporization can be calculated, in kj/mol. H vap = -m R = ( 3886 K) (8.31 J/mol K) = J/mol = 32.3 kj/mol This value compares favorably with the accepted heat of vaporization of methanol, 34.4 kj/mol. ln Vapor Pressure vs. Reciprocal of Kelvin Temperature Chemistry with Calculators 10-3 T

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