Third Virial Coefficient for Air-Water Vapor Mixtures*

The third interaction virial coefficient Caww for a ir-water vapor mixtures is estimated in the temperature range 0 to 100 °C, by means of an approximate method based on molec ular assoc iation . The res ult s are believed accurate to within a fac tor of two. The value of CawlV can be combined with prev ious es timates of the other interaction coefficient C",m and the values of the third virial coefficient s of pure air and pure water vapor, to give a quantitative description of the equation of s tate of air-water vapor mixtures through the complete third vi rial coefficient. It is shown that the maximum total contribution from both CawlV and Caaw to the compress ibilit y factor is only of the order of parts in 10' up to 100 °C and 100 atmospheres total pressure . At -50°C, it can be shown by extrapolation of Caww that the contribution of the Caww term to the compressibility fac tor is on ly on the order of parts in 1011 .