Vapor densities of magnesiumhyphen;bearing species over magnesium oxide were determined by the transpiration method. The MgO reacted with water vapor present in the carrier gas to form gaseous Mg(OH)2. The standard free energy of formation in calories per mole is given over the temperature range from 1660deg; to 2010deg;K by the equation Dgr;Gfdeg; = mdash;168 600+49.8T cal molemdash;1. Under conditions where water vapor is minimized, a second vaporization process becomes predominant. It is concluded that this process is the congruent sublimation of MgO. A secondhyphen;law treatment of these data indicates Dgr;H298= 142.5+3.3 kcal molemdash;1while a third law treatment yields 145.1plusmn;0.1 kcal molemdash;1using a1Sgr; ground state for MgO. The ground state for gaseous MgO would appear to be Sgr; and most likely1Sgr;. A value of 93 kcal molemdash;1was obtained for D0for MgO based upon the secondhyphen; and thirdhyphen;law treatment of the data.
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