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Ebulliometric Determination of Vapour Pressure

Objective of this experiment is to determine the saturation temperature for various pressures of a pure substance and to calculate the correlation constants in the vapour pressure equation.



When the vapor phase of a pure liuid is in equilibrium with its liquid phase, the equality of chemical potential, temperature, pressure in both phases leads to the Clausius-Clapeyron equation:

Most vapour pressure estimations and correlation equations stem from integration of above equation. When this is done, an assumption must be made regarding the dependence of the group (ΔHv /ΔZv ) on temperature. The simplest approach is to assume that the ΔHv /ΔZv is constant and then the above equation on integration becomes (A being the integration constant:

(where T in in K, and Δ Zv = Zvap - Zliq , Z being the compressibility factor). The above equation is called Clausius-Clapeyron equation where, B is given as ΔHv /RΔZv (ΔHv = enthalpy of vaporization). A simple modification of the equation which has been widely used is known as Antoine equation:

C = Constant, T = oC. As an approximation at low pressures (less than 1 atm), if ΔZv is taken as 1.0, the latent heat of vaporization can be directly obtained from the slope of the plot of ln P vs. 1/T .

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