Cases To Which Brown's Modified Formula Is Inapplicable

When, for a given pair of substances, the deviations of the vapour pressures from those calculated from the formula are great, the values of c are not only far from constant, but their relation to m cannot be represented by such a simple formula as c = c0 + aM. This is well seen in the case of carbon disulphide and methylal, which form a well-defined mixture of maximum vapour pressure, and of chloroform and acetone, which form a mixture of minimum vapour pressure (Zawidski).

[Table

Table 29. A = Carbon disulphide; B = methylal; t = 35.17°

Table 30. A - Chloroform ; B=Acetone; t = 3517°

Influence Of Molecular Association

The variability of c is even more marked when the molecules of one of the substances are associated in the liquid state. This is the case, for example, with mixtures of benzene and ethyl alcohol, which have also been investigated by Zawidski.

At 50° the vapour pressure of benzene is higher than that of alcohol, but at 80° it is the alcohol which has the higher vapour pressure.

Table 31. A = Ethyl alcohol; B = Benzene ; t = 50o

In Fig. 32, c is plotted against m, the molecular percentage of A in the liquid, and it will be seen that the curvature is very marked.

Mathematical Investigations.

- The whole question of the relations between the composition of liquid mixtures and (a) the partial pressures of the vapours of the components, (b) the composition of the vapour, has been discussed mathematically by Duhem,1 Margules,2 Lehfeldt,3 and Zawidski4 and others. In this connection " The Phase Rule," by Bancroft,5 may also be consulted.