water molecules move from first compartment to second compartment through the semipermeable membrane. The membrane allows only water molecules to pass through it in either direction but not NaCl. The net flow of water is into the sodium chloride solution and hence increases its volume. This decreases its concentration and also creates a pressure difference between the compartments.
This pressure difference, push some of the water molecules back to the solvent side through the semipermeable membrane until an equilibrium is established. At the equilibrium, the rate of movement of solvent molecules on both directions are equal. The pressure difference at the equilibrium is called osmotic pressure (π). Thus, osmotic pressure can be defined as “the pressure Unit Unit - - - - that must be applied to the solution to stop the influx of the solvent (to stop osmosis) through the semipermeable membrane” van’t Hoff found out that for dilute solutions, the osmotic pressure is directly proportional to the molar concentration of the solute and the temperature of the solution.
He proposed the following equation to calculate osmotic pressure which is now called as van’t Hoff equation. π = CRT ------ . Here, C = Concentration of the solution in molarity T = Temperature R = Gas constant Determination of molar mass from osmotic pressure According to van’t Hoff equation π = CRT n V Here, n= number of moles of solute dissolved in ‘V’ litre of the solution. Therefore, n RT V π or π V=nRT ------ ( .
) If the solution is prepared by dissolving w B g of nonvolatile solute in w A g of solvent, then the number of moles of solute (n) is, n= w B /M B Here, M B = molar mass of the solute Substituting the ‘n’ in ( . ), we get, π π M = w V RT M w V RT ------ ( . ) From the equation . , molar mass of the solute can be calculated.
Significances of osmotic pressure over other colligative properties Unlike elevation of boiling point (for molal solution the elevation in boiling point