84 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
point of the ether-water solution rises by — ^^ , which
can easily be measured by a Beckmann thermometer.
To carry out the determination, the tube of a Beckmann apparatus is filled with water so that the level of this stands above the bulb of the thermometer, and on to it is poured a weighed quantity of ether. After the water has become saturated with ether, its freezing point is determined. A weighed amount of the substance is then introduced, and after it has dissolved in the ether, the freezing point is again determined.
As already mentioned, one condition of the experiment is that the substance must not be appreciably soluble in water. The principle of the method is a consequence of Henry's law applied to the solubility of ether vapour in water.
Homogeneous Equilibria. — Suppose we have two vessels containing ammonium chloride vapour at high tem- perature. The ammonium chloride is partially decomposed according to the equation —
NHg + HCl -^ NH4CI.
The sign "^ denotes that there is an equilibrium — that in any specified time there is as much ammonium chloride de- composed as there is formed from the products of decomposi- tion. Let the partial pressures of the three components be Ci, C% and Ca in the first vessel, and C4, Cg, and Cs in the second. Suppose, further, that both vessels are very large. If now a gram-molecule of ammonia and a corresponding quantity of hydrochloric acid be introduced into the first vessel through a semi-permeable wall under the constant pressure Ci (or C2), the work done is pv = RT = l-99r cal. for each sort of molecule.
Suppose that the two substances then combine to form ammonium chloride, which is simultaneously removed through a semi-permeable wall in such a way that the pres- sure remains constant. By the passing out of the gram-
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