The volumetric method or manometry

In a closed system, the bonding of a molecule with a sohd contributes to lowering the partial pressure of the gas and measuring the variation of this pressure is enough to access the necessary informatioa 

Generally, the number of gas molecules introduced, n, is given, either by  

If there is no solid sample in vessel B, we naturally find that  

If there is a solid sample, we generally note that  

This experiment, if conducted under different gas pressure conditions, gives us adsorption isotherms, which plot n against P at a given temperature. 

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