Determination of boiling point

Elevation in boiling point is determined by Landsberger's method and Cottrell's method. Study of elevation in boiling point of a liquid in which a non–volatile solute is dissolved is called as ebullioscopy. Hence the constant K_b is called Ebullioscopic constant

Important relation concerning elevation in boiling point.

(1) Depression in freezing point is directly proportional to the lowering of vapour pressure. ΔT_f ∝ P⁰–P

(2) ΔT_f = K_f × m

where K_b = molal elevation constant or ebullioscopic constant of the solvent; m = Molality of the solution, i.e., number of moles of solute per 1000g of the solvent; ΔT_b = Elevation in boiling point

(3)

where, K_b is molal elevation constant and defined as the elevation in b.pt. produced when one mole of the solute is dissolved in 1 kg of the solvent
w and W are the weights of solute and solvent and m is the molecular weight of the solute.

(4)

where T₀ = Normal boiling point of the pure solvent; l_V = Latent heat of evaporation in cal/g of pure solvent; K_b for water is 0.52k–kg mol^–1.

Azeotropic mixture :

Azeotropics are defined as the mixtures of liquids which boils at constant temperature like a pure liquid and possess same composition of components in liquid as well as in vapour phase. Azeotropes changes into vapour state at constant temperature and there components cannot be separated by fractional distillation. Azeotropes are of two types as described below

(1) Minimum boiling azeotrope :

For the solution with positive deviation there is an intermediate composition for which the vapour pressure of the solution is mixture and hence, boiling point is minimum. At this composition the solution distils at constant temperature without change in composition. This type of solutions are called minimum boiling azeotrope. e.g.,

H₂O + C₂H₅OH,H₂O + C₂H₅CH₂OH

CHCL₃ + C₂H₅OH, (CH₃)₂CO + CS₂

(2) Maximum boiling azeotrope :

For the solutions with negative deviations there is an intermediate composition for which the vapour pressure of the solution is minimum and hence, boiling point is maximum. At this composition the solution distils at constant temperature without the change in composition. This type of solution are called maximum boiling azeotrope. e.g.,

H₂O + HCl,H₂O + HNO₃,H₂O + HClO₄

Some azeotropic mixtures

Mixture	% composition of azeotrope	Boiling point (pressure = 1 atm)
Nitric acid-Water	68% Nitric acid	125.5°C
Acetic acid-Pyridine	65%Pyridine	139.0°C
Chloroform-Acetone	80% Pyridine	65.0°C
Hydrogen chloride-Water	79.8% Water	108.6°C

Examples :

Ex1:

The elevation in boiling point of asolution of 13.44 g of CuCl₂ in 1 kg of water using the following information will be : (Molecular weight of CuCl₂ = 134.4 and K_b = 0.52 K molal^–1)

Sol:

CuCl₂ is an electrolyte which ionise in solution as follows:

Thus, number of particles after ionisation

= 1 – α + α + 2α = 1 + 2α.

∴ van't Hoff factor (i)

= (Number of particles after ionisation / Number of particles before ionisation)

or (i) = [(1 + 2α) / 1] (On 100% ionisation α = 1)

= [(1 + 2 × 1) / 1] = 3

The elevation in boiling point (when colligative property is abnormal)

δT_b = i × K_b × m

m → Molality of solution

Ex2:

Ratio of ΔT_b/K_b of 6% AB₂ and 9% A₂B (AB₂ and A₂B both are non – electrolytes is 1 mol/kg in both cases. Hence, atomic masses of A and B are respectively:

Sol:

ΔT =

∴ m₁ (A₂B) = 60 = A + 2B

>

∴ m₁ (A₂B) = 90 = 2A + B

A = 40, B = 10