One would also infer this from existing magnetic data which are consistent with the value to be expected from a 2Σ state. Accepting the band spectrum evidence, that NO 2 is a symmetrical nonlinear molecule, it was possible to show from the equilibrium measurements that the statistical weight of the normal electronic state is 2. In the course of the calculations several points concerning the molecular structures of nitrogen dioxide and nitrogen tetroxide were investigated. The good agreement demonstrates that the entropy value obtained from the experimental measurements and the third law of thermodynamics is the correct one. The experimental entropy value obtained in this investigation, together with band spectrum data, has made possible a much better correlation of the various measurements on the above equilibria than has hitherto been possible. These values, which are the ones which should be used in ordinary thermodynamic calculations, do not include the nuclear spin entropy, R ln 3=2.183, for each nitrogen atom. From a consideration of the available data on the equilibria N 2O 4=2NO 2=2NO+O 2 in combination with spectroscopic data for the several substances, and the experimental entropy value given above, a number of quantities of thermodynamic interest have been evaluated. By applying the third law of thermodynamics to the calorimetric measurements, the entropy of the gas, which is dissociated to the extent of 16.1 percent into nitrogen dioxide at the boiling point, was found to be 80.62 cal./deg. Solid nitrogen tetroxide 240.3 to 261.90°K, log 10 P(int. The data have been represented by the equations: Liquid nitrogen tetroxide 261.90 to 294.9°K log 10 P(int. The vapor pressures of solid and liquid nitrogen tetroxide were measured with a mercury manometer by means of an arrangement in which carbon dioxide protected the mercury surface from reaction with the nitrogen tetroxide. The heats of fusion and vaporization were found to be 35 cal./mole, respectively. The heat capacity of nitrogen tetroxide has been measured from 15°K to the boiling point, 294.25°K.
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