Chemistry: Solutions: Depression in Freezing Point, Osmosis and Osmotic Pressure (For CBSE, ICSE, IAS, NET, NRA 2022)

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Depression in Freezing Point

  • Freezing point is the temperature at which the solid and the liquid forms have the same vapour pressure.
  • The freezing point of the solution is always less than that of the pure solvent.
  • There is a depression in the freezing point of the solution due to the vapour pressure of the solution is always less than that of the pure solvent.
  • Let be the depression in freezing point, Then, In dilute solution


Where, = mass of the solvent in

= Molality

The constant for a solution is known as molar depression constant or molal cryoscopy constant for the solvent. may be defined as the depression in freezing point of a solution when one mole of a solute is dissolved in 1 kilogram of the solvent.

Osmosis and Osmotic Pressure

  • The spontaneous flow of the solvent from a solution of lower concentration to a solution of higher concentration when the two are separated by a semipermeable membrane is known as osmosis.
  • The pressure that stops the flow of the solvent is called osmotic pressure.
  • Osmotic pressure may be defined as the excess pressure that is applied to the solution side to prevent the passage of pure solvent into it when the two are separated by a perfect semipermeable membrane.
  • If the two solutions have the same osmotic pressure, they are known as isotonic solutions.
  • The osmotic pressure is a colligative property.
  • It depends on the number of particles of solute present in the solution and not on their nature.
  • At a given temperature T, the osmotic pressure of a dilute solution is found to be proportional to the concentration of the solution in moles per liter.
  • = CRT, Where = osmotic pressure, R = gas constant
  • , Where = number of moles of solute present in V liters of the solution
  • = , Where w = mass of solute dissolved in V liters of the solution, Msolute = molar mass of the solute.
  • The molar masses of the solutes can be determined by measuring the osmotic pressure of their solutions.
  • This method has been widely used to determine the molar masses of macromolecules, proteins, etc. , which have large molar masses and limited solubility.
  • Their solutions have very low concentrations and the magnitudes of their other colligative properties are small to measure.
  • The osmotic pressure measurements are done at around room temperature, this method is particularly useful for determining the molar masses of biomolecules as they are generally not stable at higher temperature.
Osmosis and Osmotic Pressure

Reverse Osmosis and Water Purification

  • If a pressure is higher than the osmotic pressure applied to the solution side, the direction of flow of the solvent can be reversed.
  • The pure solvent flows out of the solution through the semi-permeable membrane. This process is called reverse osmosis.
  • It is used in desalination of sea water to obtain pure water.

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