Materials 10-mL graduated cylinder l or 2-L beaker, preferably tall-form Thermometer

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1 VAPOR PRESSURE OF WATER Introduction At very low temperatures (temperatures near the freezing point), the rate of evaporation of water (or any liquid) is negligible. But as its temperature increases, more particles contain the minimum energy required to escape to the vapor phase. Therefore, more of the liquid evaporates. When a liquid evaporates in a closed container, the molecules of the vapor exert pressure on the container walls, as any gas does. The pressure exerted in a closed container by an evaporated liquid is known as its vapor pressure. In this experiment you will measure how the vapor pressure of water changes with temperature. Materials 10-mL graduated cylinder l or 2-L beaker, preferably tall-form Thermometer Tap water Ice Procedure 1. Place 10 ml of water in a 10-mL graduated cylinder. Put your thumb firmly over the top and invert. You should have between ml of air trapped in the graduated cylinder. If not, adjust the amount of water you add before inverting. MAKE SURE YOU AND YOUR LAB PARTNER AGREE ON HOW TO READ THE VOLUME OF TRAPPED AIR. 2. See Figure 1. Fill a 1 or 2-L liter beaker with enough water so the graduated cylinder will be submerged. In the beaker, invert the graduated cylinder containing 10-mL water; make sure there is ml of trapped air. Have both lab partners check the reading. 3. Heat the system on a hot plate to about 80 C, with gentle stirring to ensure even heating. 4. Remove the beaker and graduated cylinder from the heat source, and read the volume of moist air in the cylinder to the nearest 0.1 ml and record your value in the Summary Table. Note: You may have to lift the graduate above the water level in the beaker to make this and later volume readings. This should not affect your results so long as you work quickly. Be sure you do not lift the open end of the graduated cylinder above the water level. 5. Using a 100-mL beaker, scoop out about 100-mL of hot water and replace with cool water. Stir gently to maintain a uniform temperature throughout the system. Continue the gradual cooling, recording the volume of moist air in the graduated cylinder at ~10-degree intervals, down to a temperature of ~30 C. Record these volumes and temperatures in the Data and Calculation Table #1. 6. Once the temperature has dropped below 30 C, you are ready to get a final reading. Add ice to the beaker of water and stir gently. When the temperature has fallen to about 5 C, take a final reading of the volume and temperature. 7. Record the barometric pressure. 4-5 ml trapped air Figure 1 1

2 Calculations 1. Convert your temperature reading into Kelvin (Data and Calculation Table #1) 2. The graduated cylinder was calibrated to be read in an upright position. To allow for the fact that your readings were made with the cylinder inverted, the volume should be corrected by subtracting 0.2 ml from each recorded value. Record the corrected volumes in Data and Calculation Table #1. 3. Below 5 C the vapor pressure of the water is negligible so at this point we assume that the only gas in the graduated cylinder is the trapped air. We can further assume that the pressure in the cylinder is equal to the barometric pressure. Use these assumptions and the Ideal Gas Law to calculate the number of moles, n, of air in the graduated cylinder. (Data and Calculation Table #1) 4. Use your volume measurements for each of the other temperatures and the value of n obtained in step 2 to calculate the pressure due to air at each of the other temperatures. Answers will be less than the barometric pressure, since at each of these temperatures the graduated cylinder contains both air and water vapor (Calculations Table #2). 5. At each of the temperatures, the difference between the calculated pressure (from step 3) and the barometric pressure is the vapor pressure of water at that temperature. For each temperature enter the atmospheric pressure, the pressure due to air, and the vapor pressure of water in the appropriate columns of your table (Calculations Table #2). 2

3 Names: Date: VAPOR PRESSURE OF WATER Prelaboratory Questions 1. At any given temperature, what gas(es) is(are) present in the inverted graduated cylinder? trapped airspace 2. What is the total pressure of the gas(es) trapped in the airspace? 3. State Dalton's Law of Partial Pressures. Data/Observations 1. Record your volume-temperature data in Data and Calculation Table #1, which you will find at the top of the next page. 2. Barometric pressure: Calculations: 1. Convert barometric pressure to ATM: 3

4 Data and Calculation Table #1 Temperature ( C) Temperature (K) Observed Volume, ml Corrected Volume, ml n = moles air Show Work here: 4

5 Calculations Table #2 Temperature (K) P of air, atm Pressure of H 2 O(g), atm Pressure of H 2 O(g), mm Hg Show your work here: 5

6 Analysis 1. Plot the vapor pressure of water (mm Hg) as a function of temperature on graph paper. Draw the best smooth-curve fit you can. 2. Using the data found at: plot a second curve on the same graph showing the actual vapor pressure of water vs. temperature. Results and Conclusions; Synthesis 1. Why do we call water a vapor, but air a gas? 2. Explain how Dalton s Law of Partial Pressures applies to this experiment. 3. From your experimental results, how does the vapor pressure of water vary with temperature? 4. From your knowledge of intermolecular forces, explain how the vapor pressure of a material will vary with the polarity of the liquid. 5. Explain the relationship between the vapor pressure of a liquid and the boiling point of the liquid? 6