How molecular theory gives the osmotic law without solving the motion
Einstein writes the free energy as F = -(RT/N) lg B, where B is an integral over every state of the system. He cannot compute B, but he needs only how it depends on the volume V* the particles are held in. If they move independently to a sufficient approximation, in a homogeneous liquid, with no forces on them, B is a factor J that does not depend on V* times V* to the power n, and the change of F with V* gives p = (RT/N)ν for dissolved molecules and suspended bodies alike.