A new method is presented for counting the number of distinguishable arrangements available to a mixture of molecules in a lattice model. Chain molecules are used and these are allowed to undergo molecular clustering and chemical dissociation. The resulting equation for the configurational degeneracy is shown to reduce to, the equations of Flory and Guggenheim in the appropriate limits. The Guggenheim equation is shown to be exact in one dimension; extension to higher dimensions requires allowing the lattice coordination number to increase beyond 2. The Flory equation is shown to be the low volume fraction approximation of the Guggenheim equation. The counting method developed can be used to model entropy effects associated with chemical reactions. The equation of state derived using the new configurational degeneracy term is shown to reduce to a number of well known equations. Comparison to van der Waals equation of state shows that the improved ability of the new equation to model observed behavior, results from a redefinition of the van der Waals free volume.