T H E PARTIAL VAPOUR PRESSCRES O F BEKZESE-TOLUENE AND BEXZEKE-ETHYLBENZENE hlIXTURES B P THOMAS BELL AND ROBERT WRIGHT
The deviation of the properties of a binary liquid mixture from those calculated from its composition may in general be attributed either to the breaking down of association in one or both of its constituents, or to the formation of complexes between them. It follows therefore that the more closely the constituents of a binary mixture resemble each other the greater will be the probability of the mixture rule being followed: since the more similar the constituents the smaller the chance of reaction between them. The determination of the partial pressures of a binary liquid mixture necessitates the analysis of the mixed vapour, a process which becomes difficult when the constituents resemble one another closely. The chemical analysis of a benzene-toluene or a benzene-ethglbenzene mixture is practically impossible, and the usual physical methods of analysis by means of density or refractive index cannot be applied as the values of these constants for all three substances lie very close to each other. In the present investigation the benzene-toluene and benzene-ethylbenzene mixtures were analyzed by adding them to a known excess of pure benzene and noting the depression in the freezing point, and from the depression produced the weight of toluene or ethylbenzene in the mixture could be calculated. As a preliminary, the effect of toluene and ethylbenzene on the freezing point of benzene was determined for concentrations of solute up to about 5yc. The results are given in Tables I and I1 and from the almost linear curve obtained the composition of a mixture of known freezing point. TABLE I TABLE I1 Depression of F. Pt. of Ioog Depression of F. Pt. of I O O g benzene by addition of ethylbenzene benzene by addition of toluene. Weight of toluene
Depression of F. Pt
0.778
0,415
,046 I ,320 1.63j 1.959
0.5jO
I
0.68j 0.829
2.229
0.9j8 1.085
2.496
I ,P I 0
2,788
1,375
3.098 3.390
I . 505
4.244
2.08j
I . 637
Keight of ethylbenzene
0.350 0.630 0.983 I . 176 1,343 I . 76; I . 190 2.323 2.60j 2.899 3.198
Depression of F. Pt. 0.170
0,313 0,475
0.563 0.650 0.840
0,895 I . IO0
I .2 2 3 I . 360 1.503
r88j
PARTIAL VAPOUR PRESSURES
can be found. The determinations of the partial presmres were carried out by the air-current method. Weighed bulbs, containing the pure liquid or the mixture, were placed in a thermostat a t 20' and purified air led through them at a rate of about 3ne litre per hour. The saturated air was then passed into a test tube cooled in liquid air, where the hydrocarbons were frozen and their vapour pressures reduced to zero. The exit from the cooled test tube was connected with an aspirator fitted with a manometer. The temperature of the aspirator, the barometric pressure and the manometer reading being known the volume of air aspirated was reduced to S . T. P. The total weight of the hydrocarbon vapours in the saturated air was obtained from the loss of weight of the bulbs and checked by direct neighing of the cooled test tube, the latter weight being always used in the calculation of the composition. The whole of the condensed liquid was added to a known quantity of benzene and the depression of the freezing point determined. The amount of toluene (or ethylbenzene) present can now be calculated, the benzene in the condensed vapours being determined by difference. -1more correct result may be obtained by a recalculation in xhich the weight of benzene in the condensed vapour is added to the weighed quantity used as solvent. The weight of each hydrocarbon divided by its molecular weight and multiplied by 2 2 4 0 0 gives the volume of its vapour in cc under standard conditions. The partial pressures can now be readily calculated:
V'
5-'
+ V" + 5'
-
P' Total pressure
where T" and V" are the volumes of benzene and toluene (or ethylbenzene) vapour, and V is the volume of air all under standard conditions of temperature and pressure. P' is the partial pressure due to the benzene and the total pressure is that in the last solution bulb, i. e. the barometric height less the manometer reading. A similar calculation gves the value of P" the partial pressure of the toluene or ethylbenzene. The results obtained are given in Tables 111and IV. The calculated values are made on the assumption that the mixture rule holds when partial pressures
TABLE I11 Partial vapour pressures of benzene-toluene mixtures at 2 0 ' Molecular R Molecular 7, V.P. benzene T.P.
Benzene
Toluene
100.0
0
IO0 I O
0
67.0 54.7
43 ' 4 22.7 0
0
33.0 45 ' 3 j6.7 77.3
found
74.8 74.6 48.5 40.5
34.5 18.4
calculated
found
toluene calculated
0
0
jo.0
40.7 32.4 17.0
8.5 10.8
7.3 IO. I
12.2
12.6
17.4
17.4
100.0
0
22.0
100.0
0
22.6
1886
THOMAS BELL AND ROBERT WRIGHT TABLE
IT
Partial vapour pressures of benzene-ethylbenzene mixtures a t Molecular Benzene
Molecular Ethylbenzene
100.0
0
100.0
0
57.3 26.9 13.3
42.7
0
73.1 86.7 100.0
0
100.0
V.P. found
benzene calculated
V.P. found
74,8 74.6 43.0
42.8
3.3
21.1
20.I
5.1
9.7
9.8
6,j 7.5 i .3
0 0
20'
ethylbenzene calculated
0 0
3.2 5.4 6.4
are taken as proportional to the molecular composition of the mixture. The experimental results will be seen to be in fair agreement with the theoretical, so that if complex formation takes place in the mixtures considered it must do so to a very slight extent. Physical Chemical Laboratory, Glasgozc Uniberszty, Oclober 3, 1937
