The absorption coefficient of NO2 (g) has been found to be both temperature and pressure dependent for the temperature range 299.71° to 376.52°K and pressures from 2 to 5 cm Hg, for the 5461‐Å mercury ``line''. The equilibrium constants for the reaction N2O4 (g)=2NO2 (g), at the temperatures 299.71°, 303.70°, and 308.89°K, have been evaluated by combining precise transmittance measurements for the 5461‐Å mercury line with the known vapor pressures of solid N2O4. ΔG°298.16 for the above change in state has been found to be 1.151±0.004 kcal/mole, which, when combined with the known value of ΔS°298.16, gives ΔH°298.16=13.640±0.100 kcal/mole.

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