Molecular Weight Of Substances In Solutions : Boiling Points Of Solutions

Lowering of vapour pressure is synonymous with elevation of boiling point, since the latter is the temperature at which the vapour pressure reaches atmospheric or other total pressure. If salt is dissolved in water, the vapour pressure at 100° is less than 760 mm., and it will be necessary to raise the temperature above 100° to attain that pressure, i.e., the boiling point of the water is raised by the dissolved substance.

The elevation of boiling point of a solution is often applied in the laboratory to produce a heating- bath of higher temperature than 100°. For this purpose, solutions of the very soluble salt calcium chloride are convenient. They may be boiled in iron vessels. The boiling points for given amounts of anhydrous salt are as follows:

Parts of calcium chloride per 100 parts of water	50	200	325
Boiling point	112°	158°	180°

Such high-temperature baths may replace those using oil, glycerin, or fusible metal, except at temperatures above 200°.

The molecular elevation of boiling point, E, is constant for a given solvent. It is the rise in boiling point for 1 mol of non-volatile solute in 1 kgm. of solvent.

If w gm. of substance in 1000 gm. of solvent raise the boiling point by D°, we shall have D : E = w : M, where M is the molecular weight of the dissolved substance. Hence M = E w / D (cf. the freezing-point equation: M = Δ w / D).

The values of E for a few solvents are given below.

Solvent.	Boiling point °C.	Molecular elevation of boiling point, E.
Water	100	0.52
Methyl alcohol	64.7	0.88
Ethyl alcohol	78.3	1.15
Ether	35.4	2.10
Benzene	80.2	2.57
Chloroform	61.2	3.66

E may be calculated from the latent heat of evaporation of the solvent, L_e, in a similar way to that of Δ from the latent heat of fusion. If T is the absolute boiling point,

For water: T = 100 + 273 = 373; L_e = 539;

E = 1.988 x (373)^{2 / (539 x 1000) = 0.513 (obs. 0.516).

The equations do not hold for concentrated solutions, and the molecular weights are abnormal in solutions of electrolytes.

Example. - The molecular weight of iodine dissolved in ether may be calculated from the following figures:

2.0579 gm. of iodine dissolved in 30.14 gm. of ether gave an elevation of boiling point of 0.566°.

w = 2.0579 x 1000 / 30.14 = 68.27; D = 0.566°; E = 2.10°;

M = E w / D = 2.10 x 68.27 / 0.566 = 253.3 But I2 = 2 x 126.9 = 253.8;

iodine exists as diatomic molecules, I2, in solution in ether.}

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Molecular Weight Of Substances In Solutions
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