**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.