Osmotic Pressure

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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. For example, for NaCl, , giving the total number of and ions. All other solutes we consider are undissociated with . 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 very sensitive method for determining molecular weights , particularly for polymers. For a solute concentration of g/L, the molar concentration is equal to .


An osmotic pressure of 1 atm would cause a rise of 1033.26 cm for the solution in a U-tube. To keep magnitudes to a more manageable laboratory scale, solute concentrations are limited to the range of 1-50 mg/L; 1 cm corresponds to atm or 0.73554 torr. You can measure solutions of sugar, table salt, or a random unknown. You have sufficient data then to calculate the molecular weight of the solute or, by marking the checkbox, let the program do the calculation.


Contributed by: S. M. Blinder (March 2011)
Open content licensed under CC BY-NC-SA



Snapshots 1–3: determining molecular weights for three different solutes

See the Wikipedia entry for Osmotic pressure.

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