This quantity can be calculated for each component of a binary mixture using the van Laar equations.
An extension of Henry's law which works for higher mole fractions adds a term with the same name as this quantity. This quantity can be calculated for each component of a binary mixture using the van Laar equations. One method for experimentally determining this quantity uses measurements of the deviation from ideal colligative properties. An alternative to that Mean Salt Method works only for dilute ionic solutions and relates the natural log of this quantity is related to the (*) ionic concentration, temperature, and a series of constants including the Debye screening length according to the Debye-Huckel Limiting Law. This value approaches one at high temperatures, and equals one for an ideal solution. For 10 points, name this value which is used to relate mole fractions and the fugacities of pure components to the fugacity of a mixture as well as the namesake variable to mole fraction.
An extension of Henry's law which works for higher mole fractions adds a term with the same name as this quantity.
activity coefficient (which is not the same as activity, though the two are related)