Osmotic Pressure
Osmotic Pressure
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 Utube, 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 idealgas 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 /M.
Π
Π=i[X]RT
[X]
R=0.82057
1
K
1
mol
T
i
i=2
+
Na

Cl
i=1
ΠV=RT
n
X
M
c
X
[X]
c
X
An osmotic pressure of 1 atm would cause a rise of 1033.26 cm for the solution in a Utube. To keep magnitudes to a more manageable laboratory scale, solute concentrations are limited to the range of 150 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.
9.6781×
4
10