AVOGADRO'S LAW FOR SOLUTIONS 



33 



a constant temperature T, a kilogram of dissolved 

 substance X is to be removed from, say, an aqueous 

 solution in the form of gas or vapour, and restored to it 

 again. 



The removal of the dissolved body X is to take place 

 by means of semipermeable membranes : the solution is 

 separated from the un dissolved gaseous X by the partition 

 be (Fig. 5), which only allows the gas to pass, whilst the 

 pure solvent may be supplied through the walls a b and cd, 

 on the outside of which is solvent. 

 Gas or vapour pressure and os- 

 motic pressure are to be kept in 

 equilibrium by means of two 

 pistons, above and below. Now 

 i kilogram of the dissolved body X 

 occupies under temperature T and 

 pressure P (kg. / sq. m.) a volume 

 of V cub. metres. The ratio of 

 absorption is such that this vapour 

 is in equilibrium with a solution 



that contains i kilogram of X in v cub. metres, which solution 

 exercises an osmotic pressure p (kg. / sq. m.). If now 

 both pistons be moved upwards, i kilogram of X may 

 be removed reversibly from the solution, an amount of 

 work being done by it 



PV=RT ....... (i) 



whilst an amount of work is performed against the osmotic 

 pressure, which w^e will express with the negative sign as 



Fig. 5. 



A second process now, will restore the vapour of X to' the 

 solution. First let the vapour expand to a very great 

 volume V '^ and in so doing perform work 



-iT (3) 



The so diluted vapour may now be brought into contact 



c 



