Elevation of Boiling point

Elevation of Boiling Point:

When a liquid is heated, its vapour pressure rises and when its vapour pressure becomes equal to the atmospheric pressure, the liquid boils. The addition of a non-volatile solute lowers the vapour pressure and consequently elevates the boiling point as the solution has to be heated to a higher temperature to make its vapour pressure become equal to atmospheric pressure.

If T₁ is the boiling point of solvent and T₂ is the boiling point of solution, then the difference in the boiling points is called Elevation of boiling point. It is denoted by ‘ΔT_b’.

ΔT_b = T₁ – T₂

It has been found out experimentally that the elevation in the boiling point (ΔT_b) of a solution is proportional to the molar concentration of the solution, i.e.

ΔT_b α m

Or, ΔT_b = K_b m ——- (1)

Where m is the molality of the solution and represents moles of solute in 1 kg of solvent and K_b is called molal elevation constant.

If m = 1, then,

ΔT_b = K_b

Therefore, molal elevation constant, K_b, is the elevation in boiling point produced when one gram mole of solute dissolved in 1 kg (or 1000 g) of the solvent.

Again, we know

Molality (m) = (W_A  / Mol. mass)  x (1000/W_B )

Therefore, expression (1) becomes,

ΔT_b = (K_b x W_A x 1000) / (W_B  x  M_A )

Where, W_A = Mass of solute, M_A = Molecular mass of solute, W_B = Mass of solvent, K_b = Molal elevation constant & ΔT_b = Elevation of boiling point.