How does partial pressure work?

In any gas, the pressure exerted by the gas is proportional to the number of molecules (and so to the number of moles) of that gas in the container. (This is the #n# in #PV=nRT#)

In a mixture of gases, each component in the mixture exerts a pressure that is proportional to the number of moles of that gas, as though it was the only gas present. That is, the existence of the other gases does not alter the pressure of any component.

The portion of the total pressure that is due to any one component is called the partial pressure of that component.

You can determine each partial pressure as follows:

#"partial pressure of gas A" = ("moles of gas A")/("total moles of all gases")xx "total pressure of mixture"#

(Sorry! That whole formula was supposed to fit on one line)

For example, 78% of the air is nitrogen gas. If the total pressure in the air is 100 kPa, then, the partial pressure of the nitrogen is 78 kPa.

The partial pressure of a component is the pressure that would remain if suddenly all other molecules in the mixture were to vanish, leaving only those of the one component.