When the amount of solute is increased to 30, there are an equal number of free water molecules and solute-water complexes in the simulation. Such a solution is clearly super-saturated with solute, its viscosity is very, very high, and the solute is likely well past the point of crystallization. Clearly, this example stretches the very narrow limits of our idealized simulation, which contains a much, much lower proportion of solvent to solute molecules than all real solutions, and especially those of biological interest.
This extreme example illustrates another overly simplified aspect of this simulation. If the menu choices only changed the number of solute particles in each compartment, then there should be 60 free water molecules (150 initially - 90 attached to solute). In fact, there are only 30! This two-fold discrepancy arises because our simulations set the volume of each compartment equal to the total number of particles present. Thus, all simulations have a constant number of particles, and adding solute to a compartment lowers the number of water molecules initially present by the same amount. Technically, the solutions in all the simulations are therefore Molar and not molal!
All rigorous treatments of osmosis and other chemical phenomena
are based on molal concentrations, however, and equilibria exist
between the two compartments when equal mole fractions of solute
and solvent are present in both. At low solute concentrations,
here and in nature, the two types of solutions are equivalent
because the amount of solute present does not appreciably change
the volume of the solution.