Van’t Hoff Theory of Dilute Solutions
According to this theory, osmotic pressure of a dilute solution is the pressure which the solute will exert if it were a gas at the same temperature occupying the same volume as the solution. Therefore, all gas laws are exactly applicable to dilute solutions and hence just as one mole of gas occupying 22.4liters volume at 272k exerts one atm. pressure, so one mole of any solute present in 22.4 liters volume would have one atm. osmotic pressure. From the aboves views and experimental data, Van't Hoff deduced the following laws of osmotic pressure-
Van't Hoff Boyle's Law
Osmotic pressure (π) of a solution is directly proportional to its concentartion(C) at constant temperature.
π ∝ C at constant T
If V be the volume in liters containing one mole of the solute then-
π ∝ 1/V at constant T
Van't Hoff Charle's Law
Osmotic pressure (π) of a solution is directly proportional to temperature at constant concentration.
π ∝ T at constant C
Van't Hoff Avogadro's Law
At constant temperature, solutions having same molar concentrations of different solutes have the same osmotic pressure.i.e.
π ∝ n at constant T
Combining all these laws we get-
π ∝ nT/V
or, πV = nST
Where, S is solution constant having the value 0.082 liter atm K−1 mol−1 which is exactly the same value of gas constant R.
Conditions for the validity of these laws
1. Solutions should be very dilute
2. The solute and solvent should not be interact
3. The solute should not undergo dissociation or association in soliution.
