Osmosis and osmotic pressure -
- Osmosis - is a specific kind of diffusion or is a diffusion which takes place through semipermeable membrane.
- Definition :- The flow of solvent molecules from the solvent to solution or from a less concentrated solution to more concentrated solution through a semipermeable membrane. The flow will continue till the equilibrium is attained.
Semipermeable membrane- may be defined as a type of biological or synthetic, polymeric membrane that will allow certain molecule to pass through it.
osmotic pressure - defined as the minimum extra pressure applied on the solution to stop the flow of solvent into solution through the semipermeable membrane.
It is a colligative properties expressed by π. Since It depends upon only no. of particles of solute , is proportional to molarity C concentration of solution and the given temperature T . Mathematically-
Π α C T. . . . . . . . . . . . . . . . . . (I)
Π = CRT. . . . . . . . . . . . . . . .(2)
Here π=Osmotic pressure, R = gas constant = 0.083 L bar mol-1
we know that C = Molarity = n2/V = no of moles of solute per Litre solution. n2 = no.of moles of solute So. eq. (2) may be
Π = n2/V R T ….............................(3)
ΠV = n2 RT
or ΠV = w2/M2 RT …......4) (since n2= w2/M2)
M2 =w2RT / ΠV …......................(5)
M2 =molar mass of solute w2 = quantity of solute
It is Other method of determining molar masses of solutes. Measurement of osmotic pressure method is preferred for the determination such as biomolecules proteins and polymers this is due to following the reasons
A). is carried out at room temperature which does not require heating. Heating may change the nature of the biomolecules. i.e., biomolecules are unstable at high temperature. And polymers have poor solubility.
B). Macromolecules having high molecular mass, due to this the molarity of solution is used instead of molality. Comparatively the magnitude of osmotic pressure is larger even for dilute solution
Type of solution on the basis of the magnitude of osmotic pressure or their concentrations
There are three type of solution
I). Isotonic solution : - Such solutions which have the same osmotic pressure at The same temperature is called Isotonic solution. OR defined as solutions having equal concentration to one another
2). Hypertonic solution- If a solution has more osmotic pressure than other solution. It is called hypertonic solution. Or if a solution having high concentration than that of other this solution is called hypertonic solution
3). Hypotonic solution - If a solution has less osmotic pressure than other . It is called hypotonic solution. Or if a solution having low concentration than that of other this solution is called hypotonic solution
-Normal saline water –0.9% Sodium chloride (by mass/ volume (by mass/ volume) is safe intra venously. since the osmotic pressure associated with the fluid inside the blood cell is equal to it.
what will happen?
when cell is Kept in normal saline water (0.9 % NaCI solution by mass/volume) .
-No flow of solvent from cell to solution. i.e. solution is isotonic solution
when an animal cell is kept in a solution containing more than 0.9 % NaCI solution. - water will flow out of the cell which would be shrink . i.e. NaCl solution is hypertoric solution while the fluid of cell is hypotonic solutes .it is called plasmolysis.
when an animal cell is kept in salt solution having concentration less than 0.9 % NaCI solution.
-water will flow into the cells, would swell up i.e NaCl solution is hypotonic solution while cell fluid is hypertonic solution.
*importance of Osmosis
1.plant absorb water from soil through their roots due to osmosis.
2.Revival of wilted flowers
3.preservation of food/ meat by salt/ sugar
4.Edema- People taking a lot of salt or salty food develop swelling or puffiness of their tissues, a disease called edema. this is due to retention of water in the tissue cells & intercellular space on account of osmosis.
Reverse Osmosis
when the external pressure applied on the solution is more than osmotic pressure the solvent from the solution to the pure solvent. It is called Reverse osmosis.
importance of Reverse Osmosis:-
- desalination plants to meet their potable water requirement
- RO used as a water purifier
Abnormal Molar Masses
Molar masses that are lower or higher than expected values when calculated (generally using colligative properties) are called abnormal molar masses.
Abnormal molar masses depends upon the total number of moles particles either after dissociation or association of solute molecules in solvent or solution.
Dissociation of Solutes:
There are many solutes, such as organic acids, salts and bases, when dissolved in water dissociate into ions.
Example – When one mole of NaCl is dissolved into water, it dissociates into Na + and Cl - . After dissociation, there would be one mole of sodium ions (Na+) and one mole of chloride ions (Cl- ) released in solution. Thus, we get two moles of particles in the solution.
NaCl ---------> Na+ + Cl-
After the dissociation of NaCl or KCl or any other solutes which breaks into ions into water, the boiling point of water increases.
And when molar mass is determined experimentally, it always comes lower than the expected value.
Association of Solutes:
There are many solutes, when dissolved, get associated into the solution, this decreases the number of moles into the solution. This increases the molar mass of the solution than expected value, when determined experimentally, using colligative properties methods.
Example –
When acetic acid (ethanoic acid) is dissolved into benzene, molecules of acetic acid gets associated i.e. dimerize due to hydrogen bondings and number of particles reduced. This happens generally in the solvents having low dielectric constants.
This increases the molar mass than expected value when determined experimentally.
Van’t Hoff Factor:
Van’t Hoff, a Dutch scientist, after many experiments, introduced a factor known as Van’t Hoff factor after keeping in account of the increase or decrease in molar masses than expected because of dissociation or association of solutes.
The Van’t Hoff Factor is denoted by letter ‘i’.
Van’t Hoff defined this factor as follows:
Where abnormal molar mass is determined experimentally and colligative properties is obtained by keeping in mind that solute (non-volatile) neither associated nor dissociated.
In the case of dissociation -
And in the case of association
Relative lowering of vapour pressure of solvent
Elevation of boiling point,
Depression of Freezing point,
Osmotic pressure of solutions,
The freezing point of benzene decreases by 0.45°C when 0.2 g of acetic acid is added to 20 g of benzene. lf acetic acid associates to form a dimer in benzene, percentage association of acetic acid in benzene will be (Kf for benzene =5.12 K kg mol- 1)Read more on Sarthaks.com - https://www.sarthaks.com/186385/freezing-point-benzene-decreases-45c-when-acetic-added-benzene-acetic-acid-associates
