Molar volume times vapor pressure

If applied pressure P is increased from condition 1 to 2, then a(pyjpj) = V (Pi — PSi/RT, where molar volume is V, the gas constant is R and temperature is T. From this, e.g., for water, a 1000-fold increase in P only approximately doubles saturated vapor pressure p. For hydrocarbons, p could be doubled by a lower pressure increase, in the order of 150 times or so however for moderate pressures, a tenfold increase in P even here only increases p by some 5%. Hence, for most practical situations, vapor pressure of a liquid can be considered as independent of applied pressure. Vapor-free liquid may need chemical potential represented differently (possibly by work done). 

In the above, nj denotes the molar amount of the IL on the column, R represents the universal gas constant, and PjjJ(T) is the vapor pressure of the sample at oven temperature, vf denotes the retention volume, which is a recalculation of the retention time. The second addend accounts for the nonideality of the gas phase. The second virial coefficient of the sample can be found from tables or can be calculated. The liquid volume of the sample Vq or its reciprocal value density has been tabulated for many substances. 

When the second drum is heated, the liquid molar volume increases again, and at the same time the vapor molar volume decreases together with the pressure increase. Closely below the critical point, the phenomenon of the critical opalescence... 

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