Vapor Pressure and Molecular Weight of a Pure Liquid Purpose The purpose of this lab is to measure the enthalpy and entropy of vaporization of n-pentane. You will also determine the molecular weight of pentane. A secondary purpose is to familiarize you with the operation of a vacuum line. Introduction Vapor pressure and enthalpy of vaporization are important properties of a volatile substance. Vapor pressures are needed to understand distillation and other separation techniques. The enthalpy of vaporization is necessary to predict the change in boiling point with pressure. The enthalpy and entropy of vaporization are needed to calculate the enthalpy, absolute entropy, and Gibb's free energy of formation of gaseous substances. These thermodynamic variables are also important measures of intermolecular interactions. Vapor pressures are most accurately measured with an isoteniscope, Figure 1a. A small bulb holding the substance is attached to a open-end manometer. The sample bulb and the open-end manometer are both held in a constant temperature bath. The vapor pressure of the liquid is measured by adjusting the pressure in the ballast bulb so that the level of the manometer is the same in both arms. Then the pressure in the sample bulb is the same as in the ballast bulb, and the pressure of the ballast bulb is then recorded. The advantage of the isoteniscope is that the liquid and vapor of the sample are both kept at the constant temperature. pressure transducer pressure transducer ballast to vacuum to vacuum
sample open-end manometer constant temperature bath
sample constant temperature bath
a. Isoteniscope
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.
Vapor Pressure
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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
