An increase in pressure inside the tube increases the escaping tendency of water molecules from within the tube, since their escape through the membrane lowers the pressure. Water initially flows into the tube because the escaping tendency of H₂O molecules of the solution inside is less than that of pure water outside. This inflow of water builds up a hydrostatic head of pressure in the tube, which in turn raises the escaping tendency of H₂O molecules within the tube. When the pressure is high enough, inward flow is matched by outward flow, and a new equilibrium results. This equilibrium pressure is known as osmotic pressure. The more solute molecules or ions in solution, the higher the osmotic pressure must be to block the inward flow of water molecules.

As before, the proper approach to the problem is to set up expressions for the way in which molar free energy or escaping tendency of water molecules depends on concentration and pressure in a solution, and find the conditions under which these two opposing effects exactly cancel. The result for dilute solutions is that the osmotic pressure necessary to balance flow across a membrane is related to the molarity of the solute particles on the side of the membrane to which pressure must be applied:

C_A= molarity of A = moles of A per litre of solution

P = C_ART = osmotic pressure