Passage III

When a volatile liquid is in a closed container, the liquid at the surface is constantly evaporating (forming a vapor) and condensing (reforming a liquid). At equilibrium (when the rate of evaporation equals the rate of condensation) the pressure exerted by the vapor is called the vapor pressure, measured in millimeters of mercury (mm Hg). Students did the following experiments to study vapor pressures and boiling points.

Experiment 1

A 5 mL sample of water was placed in a flask, then heated until it boiled. A 2-hole stopper containing temperature and pressure sensors was inserted into the flask. The temperature of the liquid and its vapor pressure were recorded by computer as the flask was cooled in an ice bath to 0℃. The procedure was repeated using ethanol and diethyl ether (see Figure 1) .

Figure adapted from. Henry F. Hoytzclaw, Wlliam R. Robinson, and William H. Nebergall, College Chemistry With Qualitative Analysis, 7th ed. @1984 by D. C. Heath and Company.

Experiment 2

A beaker containing 100 mL of water was heated at atmospheric pressure (760 mm Hg) . After the water had boiled for 1 min, the temperature was recorded. The procedure was repeated using ethanol and diethyl ether (see Table 1) .

Experiment 3

A 100 g sample of water was placed in a flask and 0.1 mole of a nonvolatile substance was dissolved in the water at room temperature (20°C). The vapor pressure was measured for this solution, and for solutions containing 0.2 mole and 0.3 mole of the substance. The procedure was repeated using ethanol and diethyl ether (see Table 2).