Make your own free website on Tripod.com
Title: Vapor Pressure and Enthalpy of Vaporization of Water

Purpose: To determine the vapor pressure of water at various temperatures and to use this information to
calculate the enthalpy of vaporization of water.

Procedure:

Fill a 10 mL graduated cylinder about 2/3 full of water. Close the top with your finger and quickly invert and
lower the cylinder in a tall-form beaker half filled with water. Add water to the beaker until the water level
extends above the cylinder.
Use a ruler to measure (in mm) the difference in height between the top of the water in the beaker and
the top of the water in the cylinder, h.
Heat the assembly with a Bunsen burner until the temperature is about 80C. The air inside the cylinder
should not expand beyond the scale on the cylinder. If it does, remove the cylinder and start again with a smaller
initial volume of air. Record the temperature and the volume of air (+/- 0.01 mL) in the cylinder. Be sure to
continuously stir the water in the beaker to ensure an even distribution of heat.
Cool the beaker until the temperature reaches 50C. Record the temperature and volume of gas in the
cylinder every 5C. You may add some ice or ice water to the beaker to speed up the cooling slightly, but try to
keep the volume of water in the beaker about the same.
After the temperature has reached 50C, cool the beaker rapidly to about 0C by adding ice. Record the
gas volume and temperature at this low temperature.
Record the barometric pressure in mmHg.

Calculations:

1. There is a small error in the measurement of the volume of air caused by using the upside-down
graduated cylinder because the meniscus is reversed. Correct all volume measurements by subtracting
0.20 mL from each volume reading.

2. Calculate the total pressure of the gas in the cylinder from the barometric pressure and the difference in
water levels between the top of the water in the beaker and the top of the water inside the flask, h. The
pressure inside the cylinder is slightly greater than the atmospheric pressure. This increased pressure
can be calculated by using the measured difference in water depth, h, and multiplying by the conversion
factor that the pressure exerted by 1.00 mmHg is the same as that exerted by 13.6 mm water.

3. Calculate the moles of trapped air by using the volume of air present near 0C and the ideal gas
equation. At this low temperature we are assuming that the vapor pressure of water is negligible, so
almost no water vapor is present in the cylinder.

4. For each temperature between 50-80C, calculate the partial pressure of air in the cylinder.

5. Calculate the vapor pressure of water at each temperature.

6. Plot the ln of the water vapor pressure versus 1/T on the horizontal axis. Draw the best fitting straight
line through the points. Determine the slope of the line and calculate the value of the heat of
vaporization of water. Compare this to the reported value.

Discussion Questions:

1. In calculation 3, you were instructed to assume that the water vapor pressure at 0C is zero. Explain in
some detail why it is necessary for your subsequent calculations to make this assumption.

2. Explain in detail how the above assumption affects the actual values of the vapor pressures that you
calculate in the subsequent steps of your analysis.

3. In step 2 of the analysis, you were instructed to make a correction for the pressure inside the inverted
graduated cylinder. Explain why this correction must be applied.