In this experiment, a bubble of air below the surface of water expands and contracts as the temperature of the water changes. There are two processes involved in the volume change: the bubble expands and contracts according to the Ideal Gas Law, and the amount of water vapor in the bubble changes as the vapor pressure of water changes. If the water is at atmospheric pressure, the total pressure on the bubble is measurable with a barometer, and it is assumed that the temperature remains constant during the experiment. The total pressure is also given by Dalton's Law of Partial Pressures, where the pressure from water vapor adds to the pressure from air molecules to give the total pressure. Dalton's Law becomes: Patm = Pwater vapor + Pair. The number of moles of air does not change during the experiment. The moles of air can be calculated from the volume of the bubble at a temperature where water vapor pressure is near zero. For any other set of conditions inside the bubble, the moles of air can be redetermined from the Ideal Gas Law since the volume and temperature are known. The vapor pressure of a liquid increases with increasing temperature. Heat is absorbed by the liquid when it vaporizes, whether it is at its boiling point or some lower temperature. This heat is its Enthalpy of Vaporization. At any temperature where the liquid is in equilibrium with its vapor, the vapor pressure is given by the Clausius-Clapeyron equation: lnP = -ΔHvap / R * (1/T) + C. If the vapor pressure of water can be measured at several temperatures, this equation can be used to obtain the Enthalpy of Vaporization. This equation has the form of a straight line (y = mx + b), where the values of ln(P) are plotted against the values of (1/T), and the slope of the line is (-ΔHvap / R). The slope's units will be Kelvin. The equation can be determined from the data by Linear Regression. An overview of the procedure is as follows: Trap a bubble of air inside an inverted 10 mL graduated cylinder (readable to 0.05 mL) in a beaker of water. Read the volume of the bubble as it cools from about 80°C to about 50°C. Cool the water, cylinder, and bubble to near 0°C to get the volume of air alone. Since this value goes into every other calculated value, it is essential that it is measured carefully and at thermal equilibrium. An overview of the calculations is as follows: Correct each volume measurement for the inverted meniscus (deduct 0.20 mL). Calculate the moles of air from the 0°C corrected volume and the ideal gas law equation (n = PV / RT). Using an organized table or computer spreadsheet, calculate the pressure of the air at each temperature (Pair = nair RT / V), and the vapor pressure of water from Dalton's Law. Graph ln(Pwater vapor) vs. (1/T) and calculate the Enthalpy of Vaporization from the slope of the line.