1.6 Real system and Raoult's Law
In reality, a liquid-vapor system does not follow the linear correlation expressed by the Raoult law, but more likely, the pressure-composition correlations are represented by curves (green in the here below picture).
The Raoult law can be modify to taken in consideration the real gas behaviour by introducing a coefficient, called activity coefficient, as shown here below:
PA = gA xA PAv
PB = gB xB PBv
where gA and gB depend on temperature, pressure and composition:
gi= gi (T, P, xi)
For gi > 1 the deviation is positive and the curves will look like in the above figure with their bending to the top;
For gi < 1 the deviation is negative and the curves will have their bending to the bottom:
A system with gi = 1 is an ideal system.
Actually it is always possible to use the linear version of the Raoult's law when working at higher range of composition. In this case, as it is shown in the graph, the Raoult line coincides with the tangent to the curve of the real system.