The simple Flory-Huggins model can be combined with the lattice gas as was
done by Tompa and others. Here the same method is used to obtain the van der
Waals equation of state for a binary gas-liquid system containing a solute con-
sisting of segmented molecules. The simplified equation of state developed here
is useful for the study of the critical properties of such systems, in particular, the
dependence of the various parameters on the chain length of the molecules.