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Co f o r H2So4(g) . Constants in Equations P B. P a r t i a l Molal P r o p e r t i e s I)IV. Calculation of P a r t i a l P r e s s u r e s . t i a l p r e s s u r e s f r o m -50 t o 400" C at t h i r t y -s i x compositions between 10 and 100 weight-percent acid. A. T r i a l Calculations2 3 Tables and g r a p h s a r e provided which give the above p a r --1 - I, INTRODUCT'IOiuThe vapor phase over sulfuric acid solutions is composed of water and sulfuric acid, together with sulfur trioxide f r o m the d i ssociation of the acid:In principle, the distribution of these t h r e e components at various t e m p e r a t u r e s and a c i d concentrations can be d e t e r m i n e d by e i t h e r (a) experimental p a r t i a l -p r e s s u r e m e a s u r e m e n t s p a r t i a l p r e s s u r e s f r o m liquid-phase thermodynamic data. 4 E x p e r i m e n t a l difficulties notwithstanding, the total vapor p r e ss u r e of sulfuric a c i d , which i n m o s t c a s e s is due e n t i r e l y t o the p a r t i a l p r e s s u r e of H 0, was t h e subject of n u m e r o u s investigations between 2 1845 and 1923, Greenewalt, reviewed 19 s e p a r a t e v a p o r -p r e s s u r e d e t e r m i n a t i o n s . s u l t , b a s e d e s s e n t i a l l y on the m e a s u r e m e n t s of Burt' and of Daudt, is the accepted s t a n d a r d that a p p e a r s in todayo s r e f e r e n c e w o r k s . (1) pure-component data for two liquids and t h r e e g a s e s and G o ) , and (H;98' '"298 P -o r '298 ( 2 ) p a r t i a l m o l a l data for binary solutions (H298, Because of t h i s , no c o r r e c t i o n h a s been 11.. DERIVATION O F EQUATIONSA, P a r t i a l -P r e s s u r e Euuation F o r any component in a multicomponent m i x t u r e at equilibrium, the p a r t i a l molal f r e e energy of the vapor i s equal to the p a r t i a l molal f r e e e n e r g y of the liquid:(2) If the p r e s s u r e i s low enough s o that the vapor a c t s as a perfect gas, Equation (3b) holds at any t e m p e r a t u r e and composition. The p a r t i a l p r e s s u r e under consideration is given by the sum of two t e r m s : a pure-component t e r m , -AF"JR'L a function of. t e m p e r a t u r e only; and a n activity t e r m y In a , a function oi both t e m p e r a t u r e and composition.In o r d e r t o evaluate the p r e s s u r e y e a c h t e r m m u s t be r e l a t e d to its s t a n d a r d -s t a t e value.The pure-component term is evaluated as foPPows : This i n t e g r a t e s t o give----Any coefficient 01 ;abr;cript shown a s 298 i s actually computed a s 2 9 8 -1 5 ' K (25' C ) . I --The activity t e r m i s evaluated in a. s i m i l a r m a n n e r :wheres{ [98 298 298 Combining Eqs. (3b), (5), and, ( 7 ) , we obtainThe evaluation of G ( T ) m a y be m a d e by using heat-capacity 3 functions in a f o r m given in the l i t e r a t u r e . F o r the g a s , we haveThis gives, upon combining G 1 9 G2' and G3(T): m a y be determined as a function of t e m p e r a t u r e , a s follows.The equilibriu...