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
.
Contributed by: S. M. Blinder (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
Snapshots 1–3: determining molecular weights for three different solutes
See the Wikipedia entry for Osmotic pressure.
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