Chemistry: Solutions: Colligative Properties: Relative Lowering of Vapour Pressure (For CBSE, ICSE, IAS, NET, NRA 2022)

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Colligative Properties

  • There are certain properties of dilute solutions which depend only on the number of particles of solute. Such properties are called colligative properties.
  • There are four colligative properties such as: relative lowering of vapour pressure, elevation in boiling point, depression in freezing point and osmotic pressure.

Relative Lowering of Vapour Pressure

According to Raoult՚s law for solutions containing non- volatile solute

In a dilute solution , Therefore the term can be neglected in the denominator.

The above expression can be used to determine the molecular mass of the solute B, with relative lowering of vapour pressure of a solution of known concentration and molecular mass.

Elevation of Boiling Point

  • The boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure.
  • The vapour pressure of a pure solvent is always higher than that of its solution. So, the boiling point of the solution is always higher than that of the pure solvent.
  • Let ΔTb be the elevation in boiling point and Δp be the lowering in vapour pressure.

Then, ΔTb α Δp α XB or ΔTb = KXB

Where K = proportionality constant XB = nB / nA + nB

In a dilute solution,

Substituting the value of in the equation,

The mass of the solvent in kilograms the term is molarity m, So,

  • The constant is called as molar elevation constant for the solvent.
  • is defined as the elevation in boiling point when one mole of a solute is dissolved in one kilogram of the solvent.
  • is expressed in degree per molality.

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