Final answer:
The expression for the activity of zinc in an FCC solid solution as a function of the mole fraction of zinc is a_Zn = x_Zn.
Step-by-step explanation:
The question is asking for the expression that describes the activity (a_Zn) of zinc in a face-centered cubic (FCC) solid solution as a function of the mole fraction of zinc (x_Zn). In many solid solutions, particularly those that follow Raoult's law or Henry's law in the limit of dilute solutions, the activity of a component is proportional to its mole fraction within the solution. Therefore, the correct expression for this would be linear with the mole fraction:
a_Zn = x_Zn
None of the other options (a_Zn = (x_Zn2) / a_Zn, a_Zn = 1 / x_Zn, a_Zn = x_Zn2) reflect Raoult's or Henry's law for the activity in solid solutions correctly. Thus, the accurate answer is B) a_Zn = x_Zn.