The produced solid is not stoichiometric

Under certain conditions of pressures and temperature, the formed sohd B can present a distance to stoichiometry with respect to gas or dissolve gas significantly. We assume that only one gas is produced with molar mass Mq. Take s as the amount of gas that remains fixed per mole of B. We will quote as an example the dehydration of calcium sulfate dihydrate (gypsum) into plaster, which is, in fact, a sohd solution of water in calcium sulfate whose composition varies practically from 0 to 0.66 and which takes value of 0.5 under normal conditions of temperature and moisture from where the name of semi-hydrate comes, which is sometimes given to this solid solution. 

In such a case, the mass change is not completely representative of the extent of the reaction indeed at t time, the mass of the solid is given by  

Remark.- It is important to note that e varies with the thermodynamic condihons of pressures and temperature it means that the smdy of the influence of these variables on the reaction rate cannot be made by a simple comparison of the curves of mass change but it is imperahve to remrn in each case to the fractional extent and the rate. 

The problem becomes complicated if we cany out decompositions at variable temperature or gas pressures during the experiment, because decomposition 

Remark.- At the end of the reaction, the fractional extent reaches value of 1 and the final mass is nif, such that  

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