Vapor pressure of pure components

An ideal gas obeys Dalton s law that is, the total pressure is the sum of the partial pressures of the components. An ideal solution obeys Raoult s law that is, the partial pressure of the ith component in a solution is equal to the mole fraction of that component in the solution times the vapor pressure of pure component i. Use these relationships to relate the mole fraction of component 1 in the equilibrium vapor to its mole fraction in a two-component solution and relate the result to the ideal case of the copolymer composition equation. 

Pj°(T) equilibrium vapor pressure of pure component i activity coefficient of component i. 

Component a making up a liquid phase (L) in contact with a gas phase (G) forms a two phase system. In the equilibrium state, the chemical potential of component a in the gas and contacting phase are equal. The equilibrium saturated vapor pressure of pure component a in the gas phase over the pure liquid phase a can be designated with p. Using the expression for a perfect gas, Eq. (4-1), for the chemical potential of a, one gets an expression of the chemical potential of component a in liquid a, p (L), in the equilibrium state ... 

In other words, the vapor pressure of Component 1 of the system is equal to the vapor pressure of pure Component 1 times the mole fraction of Component 1 in the system. To extend the residual life of a volatile component in the deposit, we may admix either an inert ingredient or an ingredient of low volatility to the desired mole fraction. Further, the material used to reduce the rate of loss of Component 1 should have as low a molecular weight as practicable so as to have a relatively large mole fraction in the mixture. 

An indication of whether or not the above condition for ideality is met is obtained from the vapor pressure of the solution. At a given temperature, the vapor pressure of a pure liquid is a measure of the ability of molecules to escape from the liquid to the gas phase. By studying the vapor pressure of a solution as a function of its composition at constant temperature one may assess the solution s ideality or its degree of departure from ideality. For an ideal solution, the tendency of molecule A to escape is proportional to its mole fraction, that is, to its concentration expressed in terms of the fraction of molecules which are of type A. The proportionality constant must be the vapor pressure of pure component A because this vapor pressure is reached when the mole fraction is unity. This result is Raoult s law, which is expressed mathematically as... 

Estimate the vapor pressure of the two components in a regular solution for which cAh/(RT) = 1 and = 0.4 given that the vapor pressure of pure component A is 15.0 kPa and that of pure B, 20.0 kPa. Also calculate the Raoult law activity coefficients. Repeat the calculation for the case that cAh/(RT) = -1. 

Fig. 1.9 Vapor pressure for a hypothetical regular solution for which cAh(RT = 1 plotted against the mole fraction of component B. The vapor pressure of pure component B is 26.7 kPa, and that of component A, 20.0 kPa. The broken lines show Raoult law behavior.

If the overall mixture composition is less than that of the liquid phase, then a vapor-liquid envelope is observed as the pressure is further increased. The vapor-liquid envelope eventually intersects the pressure axis at the vapor pressure of pure component 1, since Ti is less than the critical temperature of... 

If the effect of total pressure on the liquid fugacity is negligible in the neighborhood of the vapor pressure of pure component /, then... 

Vapor pressure of pure component 1 Vapor pressure of pure component 2 Second molar virial coefficient of component Second molar virial coefficient of component Second molar cross virial coefficient Molar volume of pure liquid component 1 Molar volume of pure liquid component 2... 

Partial pressure of component n Vapor pressure of pure component n... 

Po partial vapor pressure of pure component, Pa Q heat rate, W R gas constant, J/(kg K)... 

P = vapor pressure of pure component 1 at temperature T R = gas-law constant... 

Pj vapor pressure of pure component, species j q heat flow, pcu/sec, or... 

Figure 1.4 gives data for a mixture of four hydrocarbons. Included in the data are constants A, B, and C) for the Antoine equation, a correlation that allows the calculation of the vapor pressure of pure components as follows ... 

The column pressures are set using the vapor pressures of pure components and liquid compositions in the refiux dmm x j at 320 K (so that cooling water can be used in the condenser). 

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