Stoichiometric mixtures and progress

We say that two reactants, m and k, are in stoichiometric proportion if they have the same stoichiometric abundance (see section 1.3.1), which means their amounts are such that  

In closed system, if these conditions are met at a moment of reaction they are met at all times, especially at the initial moment. 

The same ratio as [1.36] is obtained for the concentrations by dividing the amounts of substance by the same volume  

By defining a ratio, i]c, we still have the right to write the initial amount of a product as  

In the case of two products m and k of the reaction that are in stoichiometric proportion, their amounts satisfy equation [1.36], As they were already in these proportions at the initial moment we obtain  

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