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 1aFigure 1bFigure 1c

MATERIALS

computer / two 125 mL flasks
Vernier computer interface / Methanol or ethanol
Logger Pro / Ring stand and clamp
Vernier Gas Pressure Sensor / 600 mL beaker
Vernier Temperature Probe
rubber-stopper assembly
plastic tubing with two connectors

PRelab assignment

  1. Why is waiting to see liquid at the open fitting important?
  2. Read 13.12 – 13.14 in your textbook.

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.

  1. 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.
  2. Plug the Gas Pressure Sensorinto CH1 and the Temperature Probeinto 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!
  3. Obtain a rubber-stopper assembly with a 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 Figures1b and 1c:

  1. Make sure the stir bar is spinning.
  2. 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.
  3. 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.
  4. 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.
  5. Click afterthe 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.
  6. 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.

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:

    where ln P is the natural logarithm of the vapor pressure, Hvapis 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):
    the slope, m, should be equal to –Hvap/R. If a plot of ln Pvs. 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?

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.

Chemistry with Vernier: Vapor Pressure of Liquids10 - 1

2/15/2011 12:10 PM10 Vapor Pressure vMEA