Elevation of boiling point of dilute solutions and Cottrell's Method

Elevation of boiling point of dilute solutions

The boiling point of a pure liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure. Since the vapour pressure of a solution is always lower than that of the pure solvent, it follows that the boiling point of a solution will always be higher than of the pure solvent.

In the Fig., the upper curve represents the vapour pressure - temperature dependance of the pure solvent. The lower curve represents the vapour pressure - temperature dependance of a dilute solution with known concentration. It is evident that the vapour pressure of the solution is lower than that of the pure solvent at every temperature. The temperature T^o gives the boiling point of the pure solvent and T the boiling point of the pure solution. This is because at these temperatures (T^o, T) the vapour pressures of pure solvent and solution becomes equal to the atmospheric pressure.

The elevation of boiling point = ∆T_b = T - T⁰

Elevation of boiling point is found directly proportional to the molality of the solution (or) inturn the number of molecules of solute. Also it is independent of the nature of the solute for a non-volatile solute. Hence, boiling point elevation is a colligative property.

Thus it may be written as

∆T_b prop to m

Determination of molecular weight from boiling point elevation

By measuring the boiling point elevation of a solution of a known concentration, it is possible to calculate molecular weight of a non-volatile non-electrolyte solute.

∆T_b prop to m

∆T_b = K_b m

The proportionality constant Kb is characteristic of the solvent and it is called the molal boiling point elevation constant or ebullioscopic constant. It is defined as the elevation of boiling point of one molal solution.

When n₂ moles of the solute is dissolved in W₁ kg of the solvent, the molality is given by n₂/W₁.

∆T_b =  Kb W2   /  M2W1

Since W₂, is the weight of the solute, we can calculate the molecular weight of the solution using the following expression.

M2 = Kb . W2 / ∆T_bW₁

Molal Elevation (Ebullioscopic) constants (One mole of solute per 1000 grams of solvent)

Solvent       B. Pt K       Kb (K.kg.mole^-1)

Water          373.00        0.52

Benzene      353.10        2.57

Methanol    337.51        0.81

Ethanol       351.33        1.20

Carbon tetra chloride     349.72        5.01

Chloroform 334.20        3.88

Acetic acid  391.50        3.07

Acetone      329.15        1.72

Carbon disulphide         319.25        2.41

Phenol        455.10        3.56

Determination of elevation of boiling point by Cottrell's Method

The apparatus (Fig.) consists of a boiling tube (a) which is graduated and contains weighed amount of the liquid under examination. An inverted funnel tube (b) placed in the boiling tube collects the bubbles rising from a few fragments of a porous pot placed inside the liquid. When the liquid starts boiling, it pumps a stream of a liquid and vapour over the bulb of the Beckmann thermometer (f) held a little above the liquid surface. In this way, the bulb is covered with a thin layer of boiling liquid which is in equilibrium with the vapour. This ensures that the temperature reading is exactly that of the boiling liquid and that superheating is minimum. After determining the boiling point of the pure solvent, a weighed amount of the solute is added and procedure is repeated for another reading. The vapours of the boiling liquid is cooled in a condenser (C) which has circulation of water through (d) and (e). The cooled liquid drops into the liquid in (a).