TABLE OF CONTENTS Equation (8-2.1) gives the rigorous, fundamental relation for vapor-liquid equilibrium. Equations (8-2.2), (8-2.3), and (8-3.1) are also rigorous, without any simplifications beyond those indicated in the paragraph following Eq. (8-2.1). Substitution of Eqs. (8-2.2), (8-2.3), and (8-3.1) into Eq. (8-2.1) gives
where
For subcritical components, the correction factor 
is often near unity when the total pressure P is sufficiently low. However, even at moderate pressures, we are nevertheless justified in setting 
if only approximate results are required and, as happens so often, if experimental information is sketchy, giving large uncertainties in ?.
If, in addition to setting 
, we assume that ? i = 1, Eq. (8-4.1) reduces to the familiar relation known as Raoult's law.
In Eq. (8-4.1), both ? v i and ? i depend on temperature, composition, and pressure. However, remote from critical conditions, and unless the pressure is large, the effect of pressure on ? i is usually small.
