Osmosis and Osmotic Pressure :-
Spontaneous flow of solvent from dilute solution (it may be pure solvent) into the concentrated solution through a semi-permeable membrane is called osmosis. Semipermeable membranes are the sheets, which allow the passage of only solvent molecules through them e.g. egg membrane, goat's bladder, cell membrane are natural semipermeable membrane. Copper (II) ferrocyanide, Cu,[Fe(CN), ], warm mixture of gelatin and glycerine are common artificial semi-permeable membranes. Cellophane and parchment paper etc. are also used, Pressure required to prevent osmosis is called osmotic pressure.
The phenomenon of osmosis is illustrated. The left compartment of the apparatus contains pure solvent; the right compartment contains a solution. The two compartments are separated by a semi-permeable membrane (for example, a cellophane membrane) one that permits the solvent molecules (1) is pass through but does not permit the movement of solute molecules (2) from right to left.
The system has two different phases. At equilibrium, the height of the solution in the tube on right side is more than that of the pure solvent in the left tube by h. The excess hydro static pressure is called the osmotic pressure.
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So, the osmotic pressure of a solution is the excess pressure that must be applied to the solution to stop the passage of solvent molecules through semi-permeable membrane into the solution.
Osmotic pressure, π = hdg (approximate)
where, h = increase in level of tube of unit cross-section. d= density.
Following methods are used to measure osmotic pressure:
(1) PfeFfer's method
(ii) Berkley-Hartley's method
(ii) de Vries plasmolytic method
(iv) Morse-Frazer method
(v) Towasend's reverse osmosis method
van't Hoff Laws of Osmotic Pressure:
van't Hoff states that a substance in a solution behaves like a gas. Thus, laws for solutions given by van't Hoff are parallel to those related to gases.
(a) van't Hoff-Boyle's Law :- which states that the osmotic pressure (n) of a solution is directly proportional to its concentration (c) when the temperature is kept constant.
Osmotic pressure,Ï€ ∝ C (at constant T)
or Ï€ ∝ n/V or Ï€ ∝ 1/ V
where, n = number of moles of solute present in V liter of solution
Ï€V = constant
The osmotic pressure of a solution containing 1 mole of solute particles per litre (1M) at 0°C is 22.4 atm.
(b) van't Hoff Charles' Law (Gay-Lussac law) :- according to
which the osmotic pressure of a dilute solution is directly
proportional to its absolute temperature (T), when the
concentration of solution is kept constant.
Ï€ ∝ T at constant concentration Ï€ /T = constant
(c) van't Hoff Avogadro Law states that equal volumes of dilute solution of different solutes, having the same temperature and osmotic pressure contain equal number of molecules, i.e. isotomic solutions at a given temperature have same molar concentration.
Ï€ ∝ n
where, n = number of moles of solute per litre of solution.
(d) General Equation for Dilute Solutions is obtained by combining the above three laws
TV = nRT or π = CRT
On the basis of this analogy, van't Hoff proposed that a solute in dissolved state behave as a gas and osmotic pressure of the solution is equal to the pressure These laws fail to deal with concentrated solution due to the fact that osmotic pressure is actually related to the activity and not to the concentration of the solution.
Determination of Molecular Mass from Osmotic Pressure:
Osmotic pressure (n) is directly proportional to the molar concentration of solution and temperature
Ï€ ∝ C
Ï€ ∝ T
Ï€ = R X C X T
where R is solution constant and its value is same as that of gas constant.
Ï€ = CRT
In terms of n and L
C = n/V
Ï€ = nRT/V
where, w mass of solute
M= molecular mass of solute
M= wRT/Ï€V
Reverse Osmosis and Water Purification:
When a pressure more than the osmotic pressure is applied to the solution, the solvent may pass from solution in to the solvent through the semipermeable membrane. This type of osmosis is called reverse osmosis (RO). It is used for desalinating water.
A schematic set up for the reverse-osmosis used for desalination of sea-water. The average concentration of dissolved salts in sea water results in an osmotic pressure of about 22 atm. If sea water and fresh water are separated by a semi-permeable membrane (SPM) and a pressure higher than 22 atm is applied to sea water, pure water is forced out of the sea water enlarging the amount of fresh water on the other side of SPM. SPM is a film of cellulose placed over a suitable support. It is permeable to water but impermeable to impurities and ions present in sea water.
Note (1) Isotonic solutions have same osmotic pressure. It two solutions have different osmotic pressure, the one having lower pressure is called hypotonic solution and the other with higher pressure is hypertonic solution.
(2) Total osmotic pressure if a number of solution are present in the solution are their individual osmotic pressure.
Applications of Osmotic Pressure:
Some important applications of osmotic pressure are:
(a) Osmotic pressure is the most sensitive colligative property. Osmotic pressure method is widely used to determine molar masses of proteins polymers and other macro molecules.
(b) Osmotic presure is measured at room temperature, hence this method is particularly useful for biomolecules which are generally unstable at higher temperature.
(c) Osmosis plays a vital role in biology also. Some examples of it are
- For normal functioning of the living systems, the fluid concentration has to be maintained in the plant and animal cells.
- Movement of water from roots to the top of plants takes place via osmosis.
- Different movements of plants such as opening and closing of flowers, etc., are controlled by osmosis.
- A 9.91% solution of pure NaCl is isotonic with human red blood red cells (RBCs). Therefore, in this solution, RBCs neither swell nor undergo
plasmolysis. - A pure NaCl solution with concentration less than 0.91% is called hypotonic solution. On placing RBCs in this solution, they will swell and even burst.
- A pure NaCl solution with concentration more than 0.91% is called hypertonic solution. On placing RBCs in this solution, they shink due to plasmolysis.
- People taking a lot of salt or salty food experience water osmosis. The resulting puffiness or swelling is called edema.
- The use of salt and sugar as preservatives in pickels and jams has its basis in preventing growth of fungi and bacteria by osmosis.