Colligative property 3.

Elevation in Boiling Point:-

The boiling point of a liquid is the temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure. It is found that the boiling point of a solution is increased (elevated) as compared to pure solvent when solute is added to solvent. it is due to the decrease in vapour pressure of solution as compared to solvent on addition of solute. It is represented as:

Derivation of Expression:-

i.e. ΔT_b α Δp

according to Raoult’s law

                Δp α x_A                 (x_A = M.F. of solution)

Thus      ΔT_b = α x_A

                        ΔT_b = k x_A

ΔT_b = k M_B m

                ΔT_b = k_b X m        where k_b is a constant = k X M_B

The constant k_b is called the Molal Elevation Constant or ebullioscopic constant.  If m=1

(i.e. molality =1)               ΔT_b = k_b

Hence Molal elevation constant (Ebullioscopic constant) may be defined as the elevation in boiling point when the molality of the solution is unity. The units of k_b are K kg mole^-1.

Since ΔT_b = k_B X m (ΔT_b α m), it is cleat that the elevation in boiling point depends upon molality i.e. the number of moles of solute. hence it is a colligative property.

Calculation of Molar Mass of Solute:-

Let W_A gram of solute with molar mass M_A is dissolved in W_B gram of solvent: the molality of the solution is given by:

Substituting in  ΔT_b = k_b X m