DENSITIES OF SATURATED VAPOURS BY ROBERT WRIGHT
The densities of saturated vapours have been less frequently determined than those of unsaturated. In the method here described the experimental detail is simple but the temperature of the determination is confined to that of the boiling point of the liquid yielding the vapour. A small quantity of the liquid to be investigated is placed in a previously weighed stoppered cylindrical bulb of about IOO cc capacity. The bulb is bound by means of twine between two curved brass strips, which are soldered to two stout brass wires about 80 cm. long, care being taken that there is no direct metallic contact between the two supports. The strips should extend slightly below the bottom of the bulb, and the wires, which are soldered along the backs of the strips, should project about I cm beyond their lower ends. The ends of the wires below the bulb are joined by a spiral of fine platinum wire arched upwards so as to touch the bottom of the glass bulb. The upper ends of the wires pass through a cork carrying a short condenser, and the bulb and its support are suspended in the outer jacket of an ordinary Victor Meyer vapour density apparatus, in which is placed j o - IOO cc of the liquid under investigation. In carrying out an investigation the liquid in the jacket is boiled so that its vapour rises well above the top of the suspended bulb, which is unstoppered during the heating. The brass supporting wires are now connected to a low voltage battery and a suitable current passed through the heating spiral under the bulb. This current should be of such strength that it will evaporate the liquid in the bulb in about I j minutes. After evaporation is complete the vapour in the bulb is a t a temperature somewhat higher than its boiling point, the current is now stopped and the apparatus left for about I O minutes till the temperature of the bulb falls to that of the surrounding boiling vapour. The bulb is now filled with saturated vapour, it is removed from the heating jacket and immediately stoppered. After cooling to room temperature the stopper is loosened to admit air, and the bulb is weighed. Even neglecting the small volume occupied by the condensed vapour the bulb now contains less air than a t its first weighing, since air is prevented from entering owing to the vapour pressure of the liquid in the bulb. The total pressure in the bulb is the barometric pressure B, and if the pressure due to the vapour is P, then the volume of air replaced by P vapour = - V, where V is the volume of the bulb. This volume, multiplied B by the weight of I cc of air gives the weight of air excluded from the bulb by the vapour, and this weight must be added to that obtained at the second
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ROBERT WRIGHT
weighing, in order that the difference between the first and second weighings should correspond to the weight of the vapour. The vapour pressure of the liquid at the temperature of the weighing may either be obtained from tables or else determined directly by the following simple method. The closed end of a barometer tube is expanded into a bulb and then bent so as to lie parallel with the rest of the tube. A small quantity of the liquid under investigation is introduced into the bulb, and the open end of the tube is then fitted by means of a rubber stopper into a filter flask containing a layer of mercury. On exhausting the flask by a water pump, and slightly heating the bulb, all the air can be driven out of the barometer tube by means of t,he vapour stream. After disconnecting the pump, mercury rises in the tube, and allowing the apparatus to cool the vapour pressure may be read directly. From the weight of the condensed vapour and the volume of the flask the density of the saturated vapour can be calculated. In the table are given the saturated vapour densities (grams per cc.) of a number of simple organic compounds at their boiling points, and the corresponding molecular weights calculated from the relative densities are also given. It will be seen that the molecular weights are close to the normal values except in the case of acetic acid, which is known to be associated to a considerable extent in the gaseous state. In the case of acetic acid the supporting wires were made of silver as brass is readily attacked by the vapour of the boiling acid. The existence of a minimum in the vapour pressure curve of a binary liquid mixture is frequently assumed to indicate complex formation between the two constituents of the mixture. Such FIG.I a minimum is shown by the system chloroformacetone, and it is of interest to determine whether the vapour of such a system shows any indication of the existence of a compound in the gaseous state. For this purpose the saturated vapour density of the constant boiling mixture was determined, it being considered that the formation of complex molecules would be more probable in saturated than in unsaturated vapour.
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DENSITIES OF SATURATED VAPOURS
The experimental values found for the vapour density relative to hydrogen in two determinations were 48 and 48.3,corresponding to molecular weights of 96 and 96.6. The refractive index of the mixture was 1.41318corresponding to 22% acetone and 78% chloroform, or 36.5 Mol % acetone and 63.5 Mol % chloroform. On the assumption that there is no union between the molecules of the vapours, the composition corresponds to a molecular weight of 96.7,a value practically identical with the experimental figure. The results given in Table I therefore indicate that in the state of vapour there is no union between the molecules of acetone and chloroform.
TABLE I B. Pt.
Alcohol
78'
Benzene
80'
Toluene
110°
Chloroform
60'
Carbon tetra-chloride
75'
M. Wt. (found) M. Wt. (Cal.) Density at 760 mm. .oo162 46.4 46 46.8 . 00 I 64 80.0
79.6 96.0 94.6 119.0 118.0 158 . o
78
,00275
92
.00302
,00274 ,00298 119 I54
,00550
157 .o
Acetone Acetic acid
SbO I 18'
57.8 57.4 100.2
I 0 2 .o
Physical ChemistTy Department, Glasgow University. July 1, 103%
.00437 ,00433 .00452
58
.00213
60
.00314 .00318
.002I I
