Boiling point of mixtures

Components must be stable at boiling point of mixture (probably mn under vacuum) vacuum lower than 7—13 kPa (50—100 torr) may requite specialised (scraped-film) crystalliser, more compHcated vacuum system. 

Mixture Boiling Point of Pure Substance (°C) Boiling Point of Mixture (°C) Composition (% Water)... 

Suppose that we determine the boiling points of mixtures containing, say, 25, 50 and 75 molecules per cent, of the less volatile component, a, at a few pressures above and below 760 mm., in order to ascertain the boiling points under normal pressure with accuracy. If, then, we know the vapour pressures of each component at the three temperatures, we may calculate the theoretical vapour pressure of each mixture from the formula. 

With chlorobenzene and bromobenzene, which show no change of temperature or of volume when mixed together, and have the same critical pressure, and with methyl and ethyl alcohol, which have widely different critical pressures, the differences D -D, are certainly within the limits of experimental error in the other cases the differences are very small, and it may probably be concluded that the actual boiling points of mixtures of closely related substances may be calculated with very considerable accuracy from the... 

The relation between the composition and the boiling points of mixtures was determined only in series I. 

Temperature of still-head = l. Percentage of J in distillate. Percentage of A in mixture evolving vapour of the composition given in Column II. Boiling point of mixture referred to in Column III. = <2. A h-h... 

Temperature of still-head and boiling point of mixture = (. ... 

Data required.—If, for mixtures of any two substances, the curve representing the relations between boiling point and molecular composition has been constructed, and if the relation between the composition of the liquid mixture and of its vapour is known, the boiling point of the distillate may be read from the curve. We have seen that if the two substances are closely related we may safely assume that the boiling points of mixtures may be calculated from. the vapour pressures of the components, and that Brown s formula, taking the mean ratio of the vapour pressures for the value of the constant, c, gives the relation between the composition of liquid and vapour with fair accuracy. 

Benzene and Toluene.—In the case of benzene and toluene, the boiling points of mixtures have been found to agree very closely with those read from the theoretical curve, but the relation between the composition of liquid and vapour has not been experimentally determined. The mean ratio of the vapour pressures at temperatures between 80° and 110° is about 2 5, and we may probably assume that the relation is expressed with sufficient accuracy by the formula o b/ a = 2 5 Wb/ a ... 

Isopentane and Normal Pentane.— The corresponding data for mixtures of isopentane and normal pentane are given in the second part of Table 43, but in this case neither the boiling points of mixtures nor the relation between the composition of liquid and of vapour have been directly... 

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