## Henrys

At the other limit, as the solution becomes dilute in component A, the partial pressure curve turns into the straight line labeled Henry's law in Fig. 8.2. For this functional dependence, Eq (8.5) shows that the activity coefficient must be constant, although not equal to unity. This dilute solution behavior of a constant activity coefficient reduces Eq (8.5) to:

The product of the activity coefficient and the vapor pressure, kHA, is termed the Henry's law constant. Physically, Henry's law behavior in dilute solutions simply reflects the fact that all A molecules are surrounded by B molecules, irrespective of the concentration of A (as long as it is low). In the Henry's law limit for component A, component B obeys Raoult's law.

Exercises that apply the general partial pressure-composition formulas of Eqs (8.5) and (8.6) are part of problems 8.2, 8.10, 8.15, 8.18 and 8.19. In some of these problems, the activity coefficient is expressed by the regular-solution formulas of Eqs (7.42). The remaining exercises involve nonideal solutions with nonregular activity coefficients. 