where y% is the mol fraction of component 1 in the vapor.
This is the most commonly used form of Dalton's law. For the prediction of vapor-liquid equilibria, it is usually combined with Raoult's law to give yuc - XiPi (3-4)
This combination gives the composition of vapor as a function of the composition in the liquid with the total pressure and vapor pressure as proportionality constants. Thus, fixing the temperature and the total pressure defines ir, Pi, and the relationship between the vapor and the liquid composition. It is to be noted that the assumption of Raoult's and Dalton's laws leads to the conclusion that the relationship between the vapor and liquid composition of a given component is a function of the temperature and pressure only and is independent of the other components present. The only influence of the other components is the fact that they may be instrumental in determining the relationship between the temperature and the total pressure.
Deviations from Dalton's Law. Both forms of Dalton's law are satisfactory for engineering uses for most gas mixtures at pressures of 1 atm. or lower because deviations from the perfect-gas law are usually small in this region. However, when higher pressures are encountered and appreciable deviations from the perfect-gas law are found, Dalton's law becomes unsatisfactory. A later section of this chapter will consider methods of handling these deviations, but as a general rule Dalton's law should not be employed for cases in which the deviations from the perfect-gas law are large.
• Volatility. The term "volatility" is loosely used in the literature, generally as equivalent to vapor pressure when applied to a pure substance; as applied to mixtures, its significance is very indefinite. Because of the convenience of the term, the volatility of any substance in a homogeneous liquid will be^defined as its partial pressure in the vapor in equilibrium with that liquid, divided by its mol fraction in the liquldT^If the substance is in the pure state, its mol fraction is unity and its volatility is identical with its vapor pressure. If the substance exists in a liquid mixture that follows Raoult's law, its volatility as thus defined is still obviously equal to its vapor pressure in the pure state; i.e., its volatility is normal. If the partial pressure of the substance is lower than that corresponding to Raoult's law, e.g., that of hydrochloric acid in dilute aqueous solutions, the volatility according to this definition is less than that of the pure substance, i.e., is abnormally low. Similarly, if the partial pressure is greater than that indicated by Raoult's law, e.g., that of aniline dissolved in water, the volatility is abnormally high. The volatility of a substance in mixtures is therefore not necessarily constant even at constant temperature but depends on the character and amount of the components.
Relative volatility is the volatility of one component divided by that of another. Since the volatility of the first component of a mixture, is its partial pressure, pa divided by its mol fraction xa, and that of the second fa = pi/xi, the volatility of the first relative to the second is 13a//3b = PaXb/pbXa* When Dalton's law applies, the relative amount of any two components in the vapor (expressed in mols) is ya/yb = Pa/Pb.
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