Osmosis involves the passage of solvent molecules through a semipermeable membrane from a region of high solvent concentration to one of lower solvent concentration. The net effect of this passage is to lower the concentration of solute on one side of the semipermeable membrane while increasing the pressure on that side of the membrane. Osmosis most often involves water as the solvent. Osmosis is described in several places. Because there is a chemical driving force for osmosis, a pressure is created called the osmotic pressure [local]. When a greater pressure than this is applied through appropriate devices, it is possible to bring about reverse osmosis, a process often used to purify water.

Osmometry is a technique for using osmotic pressure to determine solute molecular weight [local]. Using visocity and osmometry to determine solute molecular weight gives rise to several definitions [local] for average molecular weight.