Osmotic Pressure
Prepared by: S. M. Blinder
Osmosis involves the selective passage of certain components of a solution through a semipermeable membrane, with exclusion of other components. It is, of course, of central significance in biological processes. Consider in this Demonstration a membrane permeable only to water, but impermeable to the solute in a water solution. The membrane is represented by a blue disk at the bottom of the U-tube, separating the pure solvent on the left from the solution on the right. Solvent will spontaneously flow through the membrane into the solution, in a (vain) attempt to equalize the concentrations on the two sides. This gives rise to an osmotic pressure, designated . For dilute solutions, the osmotic pressure, in atm, is well approximated by the van 't Hoff equation , where  is the solute concentration in mol/L,  L atm , the ideal-gas constant, and , the absolute temperature in K. The van 't Hoff factor represents the number of ions per molecule for a dissociated solute. Note that for NaCl, the mole fraction used in the osmotic pressure equation is the mole fraction of ions, which is twice the mole fraction of NaCl. The other solutes in this simulation are undissociated.
The van 't Hoff equation can be written in a form remarkably analogous to the ideal gas law: , but the underlying mechanisms for the two phenomena are completely different. Osmotic measurements provide a  sensitive method for determining molecular weights , particularly for polymers. For a solute concentration of  g/L, the molar concentration  is equal to .

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