Consider a closed containing a solid in equilibrium with its vapor. The volume of the solid is much less than that of the container. Let the partition function of the solid be given by Q-qss where Ns is the number of molecules in the solid phase. Minimize the total Helmholtz free energy at fixed N, V, and T, and show that the equilibrium condition is given by Ng=qo/q2 where Ng is the number of gas phase molecules and qg is the partition function of a gas phase molecule.