Study of hydrates with variance

Since we have only one non-stoichiometric solid phase, we will approach the study of this system by quasi-chemistiy of structure elements. However, this approach presents some difficulties. Indeed, hydrated salts are relatively conplex solids with at least three principal components the anion (itself often complex), the cation, and water. If salt admits several limiting hydrates, water molecules are not all equivalent. All these complexities require a simplification of the representation of solid. With this intention, we consider hydrated solids as pseudo-binary (see section 2.4.1) of which one of the components is the water concerned with dehydration and the other component is the skeleton of anhydrous salt or incorporates possible n molecules of water not implicated in the equilibrium under study. We will disregard specific defects related to the skeleton and thus take into account the following structure elements  

We make the assumption of the prevalent defect, which leads to three types of hydrates  

We assume perfect solutions, although the defects are not neeessarily very diluted however, they are electrically neutral. 

The equilibrium that involves interstitial mobile water molecules can be written as follows  

We consider index 1 as the property relating to the skeleton and index 2 as those relating to water in the solid. We will take the quantity s of water contained per mole of solid as a variable, but this quantity includes n moles related to the skeleton and the true variable will be the difference e- n. Then, the mole fraction of water in the solid is as follows  

---