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 .