Solutions 4

All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such properties are called colligative properties These are: (1) relative lowering of vapour pressure of the solvent (2) depression of freezing point of the solvent (3) elevation of boiling point of the solvent and (4) osmotic pressure of the solution.

the vapour pressure of a liquid increases with increase of temperature. It boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. For example, water boils at 373.15 K (100° C) because at this temperature the vapour pressure of water is 1.013 bar (1 atmosphere).

The vapour pressure increases with temperature. When a non volatile solute is added to a solvent to make a solution, its vapour pressure decreases. IN order to make the solution boil, its temperature has to be raised to equal the vapour pressure to 1atm so the boiling point of the solution is always greater than the solvent for dilute solutions the elevation of boiling point (∆Tb ) is directly proportional to the molal concentration of the solute in a solution.

Kb is called Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic Constant). The unit of Kb is K kg mol-1 It is the elevation in boiling point of a solution of unit molality.

The lowering of vapour pressure of a solution causes a lowering of the freezing point compared to that of the pure solvent the freezing point of a substance may be defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase. A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent

The proportionality constant, Kf , which depends on the nature of the solvent is known as Freezing Point Depression Constant or Molal Depression Constant or Cryoscopic Constant. The unit of Kf is K kg mol-1

Semipermeable membranes are those membranes which allow the passage of only certain molecules through it. These membranes appear to be continuous sheets or films, yet they contain a network of submicroscopic holes or pores. Small solvent molecules can pass through these but larger solute molecules cannot. These membranes can be of animal or vegetable origin and these occur naturally such as pig’s bladder or parchment or can be synthetic such as cellophane, cellulose acetate.

The movement of solvent molecules from pure solvent to solution across a semi permeable membrane is called osmosis. mvmnt of solvent molecules from a region of low conc of solution to high conc of solution solvent molecules always flow from lower concentration to higher concentration of solution.

it is the excess pressure that is applied to a solution in order to prevent osmosis or to prevent the entry of solvent molecules into it. capital pi= CRT C- concentration of soln in mol/L R- 0.0821 Latm/mol K T- absolute temp Osmotic pressure is a colligative property as it depends on the number of solute molecules and not on their identity. For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the solution at a given temperature T.

i)The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. ii)As compared to other colligative properties, its magnitude is large even for very dilute solutions. (of the order of 10^-3 atm) ii)iThe technique of osmotic pressure for determination of molar mass of solutes is particularly useful for biomolecules( proteins, polymers other macromolecules) as they are generally not stable at higher temperatures and polymers have poor solubility.

It has experimental difficulties

Two solutions having same osmotic pressure at a given temperature are called isotonic solutions. When such solutions are separated by semipermeable membrane no osmosis occurs between them. As osmotic pressures are qual, at the same temperature, the concentration will be equal.

m. For example, the osmotic pressure associated with the fluid inside the blood cell is equivalent to that of 0.9% (mass/ volume) sodium chloride solution, called normal saline solution and it is safe to inject intravenously. On the other hand, if we place the cells in a solution containing more than 0.9% (mass/volume) sodium chloride, water will flow out of the cells and they would shrink. Such a solution is called hypertonic. If the salt concentration is less than 0.9% (mass/volume), the solution is said to be hypotonic. In this case, water will flow into the cells if placed in this solution and they would swell.

Hypotonic solution refers to a solution containing a lower amount of solute in comparison to the solute concentration in other solutions, across a semipermeable membrane.

A raw mango placed in concentrated salt solution loses water via osmosis and shrivel into pickle. Wilted flowers revive when placed in fresh water. A carrot that has become limp because of water loss into the atmosphere can be placed into the water making it firm once again. Water will move into them through osmosis. Water movement from soil into plant roots and subsequently into upper portion of the plant is partly due to osmosis. The preservation of meat by salting and of fruits by adding sugar protects against bacterial action. Through the process of osmosis, a bacterium on salted meat or candid fruit loses water, shrivels and dies.

People taking a lot of salt or salty food experience water retention in tissue cells and intercellular spaces because of osmosis. The resulting puffiness or swelling is called edema.

The direction of osmosis can be reversed if a pressure larger than the osmotic pressure is applied to the solution side. That is, now the pure solvent flows out of the solution through the semi permeable membrane. This phenomenon is called reverse osmosis.

When pressure more than osmotic pressure is applied, pure water is squeezed out of the sea water through the membrane. A variety of polymer membranes are available for this purpose. The pressure required for the reverse osmosis is quite high. A workable porous membrane is a film of cellulose acetate placed over a suitable support. Cellulose acetate is permeable to water but impermeable to impurities and ions present in sea water. These days many countries use desalination plants to meet their potable water requirements.

Molecules of ethanoic acid (acetic acid) dimerise in benzene due to hydrogen bonding. This normally happens in solvents of low dielectric constant. In this case the number of particles is reduced due to dimerisation. In this case, the molar mass will be odubled

KCL dissociated into K+ and Cl- ions so the Tf will be shown for twice the no of particles Hence the molar mass will be halved

a molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass. In 1880 van’t Hoff introduced a factor i, known as the van’t Hoff factor, to account for the extent of dissociation or association. In case of association, value of i is less than unity while for dissociation it is greater than unity.

it is the ratio of observed or experimental value of collimation property to the calculated/theoretical value of the colligative property. It is used to represent the extent of association or dissociation in a solution.