Raoults law is a phenomenological relation that assumes ideal behavior based on the simple microscopic assumption that intermolecular forces between unlike molecules are equal to those between similar molecules, and that their molar volumes are the same: the conditions of an ideal solution. This is analogous to the ideal gas law, which is a limiting law valid when the interactive forces between molecules approach zero, for example as the concentration approaches zero. Raoults law is instead valid if the physical properties of the components are identical. The more similar the components are, the more their behavior approaches that described by Raoults law. For example, if the two components differ only in isotopic content, then Raoults law is essentially exact.
Comparing measured vapor pressures to predicted values from Raoult's law provides information about the true relative strength of intermolecular forces. If the vapor pressure is less than predicted (a negative deviation), fewer molecules of each component than expected have left the solution in the presence of the other component, indicating that the forces between unlike molecules are stronger. The converse is true for positive deviations.
For a solution of two liquids A and B, Raoult's law predicts that if no other gases are present, then the total vapor pressure  above the solution is equal to the weighted sum of the "pure" vapor pressures  and  of the two components.  Thus the total pressure above the solution of A and B would be
Since the sum of the mole fractions is equal to one, 
This is a linear function of the mole fraction , as shown in the graph.