The Entropy of Acetic Acid

From third law measurements, vapor pressures and vapor densities, the entropy of the acetic acid monomer at 25° and one atmosphere is 69.4±1.0 e.u. The value 68.7 is calculated from the vapor phase ethyl acetate equilibrium. For a model based upon acetone and approximately representing free rotation the entropy would be 72.7. If there is only a single potential minimum in the hydroxyl group rotational cycle, the large deficiency below the free rotation value is explained without assuming an exceptionally high potential barrier. A brief discussion of the effect of the number of potential minima and their relative depth is appended, and a possible source of error in third law measurements is suggested.