Osmotic Pressure
For the phenomenon of osmosis, a membrane separates salt/water inside a chamber from pure water in the container. Water passes through membrane from dilute to more concentrated. As water rises into tube, it creates a pressure. Eventually this pressure (osmotic pressure) prevents further passage of water through the membrane.

Osmotic pressure is force per area that prevents water from passing through membrane!

Osmosis: Solvent passes through membrane
Dialysis: Solute passes through membrane

PV = nRT
pV = nRT
p = (n/V)RT = MRT

Problem #1
Calculate molarity if solution in water (300K) has osmotic pressure of 3.00 atm.
M = p/RT = (3.00atm)/[0.0821 atm.L/mol.K)(300K)] = 0.122M

Problem #2
Calculate osmotic pressure for 0.10 M Na3PO4 at 20°C.
Since Na3PO4 ionizes into 4 particles (3 Na+1 + PO4-3), the ion concentration is 0.40 M
p = MRT = (0.40)(0.0821)((293) = 9.6 atm