Boiling Point Elevation By Non Volatile Solute

Boiling Point Elevation By Non Volatile Solute

The boiling point Tb of a liquid is the temperature at which its pressure is equal to the atmospheric pressure. When a non – volatile solute is added to a liquid, the vapour pressure of the liquid is decreased. Hence it must be heated to a higher temperature in order that its vapour pressure becomes equal to that of the atmospheric pressure. This means that the addition of a non – volatile solute to a liquid raises its boiling point.

If is the boiling point of the solvent and Tb is the boiling point of the solution, then

(Elevation of boiling point)

ΔTb α molality (m)

ΔTb = Kb × m, kb : molal elevation constant or Ebullioscopic constant

m: molality of the solution

Elevation in boiling point can be calculated as

[M1 is the molar mass of solvent is enthalpy of vaporization per gram of solvent.

For a given solvent, is constant, it is denoted by Kb and called molal elevation constant of the solvent thus

Hence

It molality m = 1, i.e. 1 mole of the solute is dissolved in 1 kg of the of the solvent. Then Kb = ΔTb. Thus molal boiling point evevation constant or ebullisocpoic constant of a solvent is defined as the elevation in boiling point of the solution which may be theoretically be produced when 1 mole of the non volatile, non electrolyte solute is dissolved in 1 kg of the solvent.

The elevation in boiling point depends only on the molality of the solute and is independent of the nature of the solute, it is therefore a colligative property.

Determination of molar mass using boiling point elevation method

It W2 grams of the solute of molar mass M2 is dissolved in W1 kg of the solvent then the number of moles of the solute dissolved in 1 kg of the solvent would be given by