Osmotic Pressure:
Osmotic pressure is the external pressure applied to the solution in order to stop the osmosis of solvent into solution separated by a semi-permeable membrane. It is denoted by the symbol ‘π’.
Determination of Osmotic Pressure Berkeley and Hartley’s method:
The principle of this method is to apply external pressure on the solution by putting pressure on the piston just sufficient to prevent the osmosis of the solvent into it. The apparatus consists of a porous pot containing copper ferrocyanide deposited on its walls which acts as a semipermeable membrane. It is fitted into a bronze cylinder, which is fitted to a piston and pressure gauze. The porous pot is fitted with a water reservoir on one side and a capillary indicator on the other side. Water is put in the porous cell while the bronze cylinder is filled with the solution, the osmotic pressure of which is to be measured. Water placed in the porous pot tends to pass into the solution through the semi-permeable membrane with the result that the level in the capillary indicator moves downwards. External pressure is now applied on the piston so that the water level in the capillary indicator tube remains constant. This pressure which is equal to the osmotic pressure is read from the pressure gauze.
Isotonic solutions: When two such solutions are separated by a semi-permeable membrane and there is no flow of solvent molecules from the solution of lower osmotic pressure towards solution of higher osmotic pressure, such solutions having same osmotic pressure are called isotonic solutions or isosmotic solutions.
Hypertonic and Hypotonic solutions: If a solution has more osmotic pressure than some other solution, it is called hypertonic. On the other hand, a solution having less osmotic pressure than the other solution is called hypotonic. Thus, a hypertonic solution will be more concentrated with respect to other solution and a hypotonic solution will be less concentrated with respect to other solution.
Law of Osmotic Pressure:
1) The osmotic pressure of a solution at a given temperature is directly proportional to its concentration. π α c ———-> (1)
2) The osmotic pressure of a solution at a given concentration is directly proportional to the absolute temperature. π α T ———-> (2)
Combining expression (1) and (2), we have
π α cT
Or, π = KcT
Or, π = (n/V) RT
Or, π V= wRT/M
Where,
π = Osmotic pressure
w = Weight of solute
V = Volume of solution
R = Universal gas constant (0.0821 lit. atm/K/mol)
T = Absolute temperature
M = Molecular mass of solute
Molar concentration (c) = n/V
Number of moles (n) = w/M
Abnormal Molecular Mass (electrolytic solution): The colligative properties of non-electrolytes which do not undergo any dissociation or association in the solution give expected or theoretical value. But in some cases, the molar masses determined by these methods do not agree with the expected or theoretical values, called abnormal molecular mass. This may be due to the association or the dissociation of the solute molecules in the solution.
For example, in benzene solvent, both acetic acid and benzoic acid exist as dimers as:
2CH3COOH ———-> (CH3COOH)2
2C6H5COOH ———-> (C6H5COOH)2
The molar masses of acetic acid and benzoic acid have been found to be nearly 120 and 244 which are double of their normal values of 60 and 122 respectively. The association of solute molecules in a solution is generally due to the hydrogen bonding between these molecules.
Molecules of certain substances (acids, bases and salts) dissociate or ionise in a solvent to give two or more particles. For example, KCI dissociates to give K+ and Cl– ions.
KCl ———-> K+ + Cl–
Since each KCl molecule dissociated into two ions, the molar mass of the salt must be about half of its normal value i.e. 37.5.
Van’t Hoff factor (i):
In 1886, Van’t Hoff introduced a factor called Van’t Hoff factor ‘i’ to express the extent of association or dissociation of solutes in solution. It is the ratio of the normal and observed molar masses of the solute, i.e.
Van’t Hoff factor (i) =Normal molecular mass/Observed molecular mass
Note:-
- In case of association, observed molar mass being more than the normal, the value of Van’t Hoff factor is less than 1.
- In case of dissociation, the value of Van’t Hoff factor is more than 1 because the observed molar mass has a lesser value.
- In case of solutes which do not undergo any association or dissociation in a solvent, the value of Van’t Hoff factor will be equal to 1 because the observed and normal molar masses will be same.