A vapour–liquid equilibrium ratio of a substance:
where x and y are the mole fractions of the substance (i). It is used to describe the complex relationship between pressure, temperature, and equilibrium for vapour- and liquid-phase compositions, and used in multicomponent distillation calculations. For mixtures of substances of similar molecular structure, such as hydrocarbons, the K-factor is dependent on pressure and temperature, and commonly presented as nomographs known as K-factor charts, or DePriester charts. These present pressure, temperature, and K-factors for various light and heavy hydrocarbons. They are used to determine the bubble point and dew points of hydrocarbon mixtures. Given a multicomponent mixture of known composition and total pressure, the bubble point temperature can be found by trial and error which satisfies the condition. The factors are either used as they stand or are converted to relative volatility values. The bubble point is found from:
Similarly, for a given vapour composition, the dew point can be found which satisfies the condition
Sensible choices of temperature or pressure can mean that only three values are required.