Osmotic pressure is the pressure generated by osmosis through a semi-permeable membrane, or a barrier that only allows certain substances to pass through.

The diagram above depicts the generation of osmotic pressure through osmosis. Water tends to diffuse towards areas of higher concentration. In this diagram, pure water diffuses into the NaCl solution because of the higher concentration of ions inside the inner container. Because of this, the amount of ions (van’t Hoff factor) influences the amount of osmotic pressure. The higher the van’t Hoff factor, the greater the diffusion of water, and the greater the osmotic pressure.

Sample Problem: Calculate osmotic pressure in a solution of 2.0M NaCl at STP. Method: First, we need to find M, the ion concentration. Notice that NaCl ionizes into two particles, Na+ and Cl-. That gives NaCl a van’t Hoff factor of 2. Multiply this by 2.0M to get 4.0M. Plug it all into the equation and solve for p:

p=MRT = (4.0)(0.0821)(273K) = 90 atm


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