Note 39 — Chapter III · OSMOTIC PRESSURE AND THE THEORY OF SOLUTION II

The molecules may have different masses in the different conditions, and the principle of the mean kinetic energy would always apply to them as they are, in the condition under observation, and not as they are in some other condition; any change in mass, in solution, for instance, would show itself in the osmotic pressure measurements (see p. 18), just as it is shown in the measurements of gases, when the gas molecules show a change in composition, as is the case with hydrogen fluoride (H2F2 ⇄ 2 HF), nitrogen tetroxide (N2O4 ⇄ 2 NO2), phosphorus pentachloride (PCl5 ⇄ PCl3 + Cl2) and other compounds.