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.


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