sample open-end manometer constant temperature bath
sample constant temperature bath
b. Vacuum line
Figure 1. Vapor pressure measurement: (a) isoteniscope, (b) vacuum line. In a vacuum line part of the vapor is at room temperature, causing a temperature gradient. A less accurate, but more common method for measuring vapor pressures is a vacuum line, Figure 1b. Vacuum lines are used in handling gases and air or moisture sensitive compounds. Inorganic chemists often use vacuum lines for synthesis and characterization. For simple compounds, vapor pressure and enthalpy of vaporization are often used for quick characterization of unknowns. Therefore, a vacuum line is an important piece of laboratory apparatus that every chemist should know how to use.
In a vacuum line, the vapor pressure of the substance is directly measured by the pressure transducer. In other words, the substance comes in direct contact with the pressure measurement device, since there is no intervening open-end manometer. Therefore, most of the vapor is at a different temperature than the liquid. This temperature gradient makes true equilibrium impossible to establish, and also the properties of the gas are different in different parts of the vacuum line. These effects cause systematic errors in the vapor pressure measurement. However, the speed and convenience of using the vacuum line method usually outweigh these errors. The procedure for the vacuum line is very simple. The sample is placed in a constant temperature bath; after the pressure stabilizes, the pressure is recorded. To give you some practice in using the vacuum line, you will also experimentally determine the molecular weight of the substance using a vapor density measurement.
Theory The relationship between vapor pressure and temperature is given by the Clausius-Clapeyron equation: p2 -∆vapHm 1 1 ln ( p ) = (T -T ) (1) R 1 2 1 The assumptions used in the derivation of eq. 1 are that the vapor behaves as an ideal gas, the molar volume of the liquid is much smaller than the vapor, and the enthalpy of vaporization is independent of temperature. For this experiment we need to relax the assumption that the vapor behaves ideally. We also need to consider the effects of the assumption that the enthalpy of vaporization is independent of temperature. Please review the derivation of the Clapeyron equation in your text. The Clapeyron