The partial-pressure concept

The partition is removed, and the hydrogen fills the entire vessel. After a period of time, the mercury levels rest in the final positions shown in Fig. 2.8(b). Since the volume available to the hydrogen has doubled, the pressure in the container has f alien to one-half its original value. (We neglect the volume of the manometer arm in this computation.) 

The interpretation of each experiment is direct. In the first experiment, the manometer read the total pressure both before and after the partition was removed  

In the second experiment, the manometer read total pressure before the membrane was removed, and partial pressure of hydrogen in the mixture after removal of the membrane  

Notethat the total pressure in the container does not change upon removal of the partition. 

It is possible to measure the partial pressure of any gas in a mixture directly if there is a membrane that is permeable to that gas alone for example, palladium is permeable to hydrogen and certain types of glass are permeable to helium. The fact that at present only a few such membranes are known does not destroy the physical reality of the concept of partial pressure. Later it will be shown that in chemical equilibria involving gases and in physical equilibria such as solubility of gases in liquids and solids, it is the partial pressures of the gases in the mixture that are significant (further confirmation of the physical content of the concept). 

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