Mole Fraction Diagram

The composition is usually expressed as the mole fraction of one component, while the vapour pressure is measured in pressure units such as Pascal or Torr. By plotting the vapour pressure as a function of composition, the diagram allows us to visualize how the vapour pressure changes with varying composition.. In mole fraction terms, 68% by mass of nitric acid corresponds to a mole fraction of only 0.378. The whole balance of the diagram will look different if you use the alternative scale.

Beware! These diagrams (and the ones that follow) only work properly if you plot the partial vapour pressure of a substance against its mole fraction. If you plot it against its mass or its percentage by mass, you don't get a straight line. Instead, you get a slight curve. This is a result of the way the maths works. You don't need to worry about this unless you come across a diagram for ideal.. yA and yB : mole fractions of the vapour components The calculation of yA through the use of the pressure of each allows the concentration of yB to be found. The total pressure can also be calculated with just one gas phase concentration and its relative partial pressure. Vapour - Pressure Composition (VPC) Diagrams