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The concept of vapor pressure of liquids is discussed in most general chemistry and physical chemistry courses. The Clausius-Clapeyron equation integrated for liquid-vapor equilibrium is introduced to describe the variation of vapor pressure with the temperature and to determine the enthalpy of vaporization. Several demonstrations involving the volatility of liquids (1-3) and methods of measuring the change in vapor pressure with change in temperature have been reported in this Journal (4-8). In all of them, the measurement of the vapor pressure was achieved by measuring the pressure of the vapors on liquids using several types of manometers.
On the other hand, many organic liquids have significant vapor pressure at room temperature and present relatively high molar absorptivity in the W region. For these materials the absorbance of the saturated vapors, which is proportional to the concentration of molecules in the vapor phase, may be used to monitor the vapor pressure of the liquid.
In the experiment described here, students test this hypothesis, and measuring the absorbance at several temperatures, they are able to determine the enthalpy of vaporization of the liquid. Using vapor pressure values reported in literature and the absorbance measured, they find the correlation between vapor pressure and absorbance. The molar absorptivity of the organic product in the vapor phase can be also estimated. This experiment is appropriate for a physical chemistry course and for undergraduate research.
Theoretical
The Beer-Lambert equation relates the absorbance (A) of a solution at a particular wavelength (lambda) with molar absorptivity (e in M sup -1 X cm sup -1 ), substrate concentration (c in M), and cell length (l in cm).
(Equation 1)
A = e X c X l
Although this relationship is used almost exclusively for dilute solutions of absorbing molecules, it is also applicable in the gas phase. Actually, in the last case the absorbance measurements are not affected by solvent background or intermolecular interactions typical of solutions.
Because most organic vapors are heavier than air, in a nearly closed UV cell containing some drops of a liquid, an...