Method of total pressure

In a gas-phase system at constant volume, the reaction is monitored through the formation of products and change of total pressure. The pressure is a direct measure in a closed system. If the partial molar fractions are known, the partial pressure can also be determined as a function of the total pressure. For a gas-phase reaction of the type like  

Using Equation 1.1 and substituting a n = no + Ava and knowing that =- f T, the partial pressure of A can be calculated  

Since the total initial pressure and pressure at time t are Pq and P, respectively, we have  

Regarding the product, only the sign is changed from (—) to (+), i.e., 

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