Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
In a hill town the air pressure is kPa. By number of molecules, dry air is nitrogen, oxygen and argon. Find the partial pressure of each gas.
Each gas in a mixture fills the whole volume. Its partial pressure is . The total pressure is the sum:
A real gas has below when the pull between molecules wins and above when their size wins.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Nitrogen. kPa.
Step 2. Oxygen. kPa.
Step 3. Argon. kPa.
Check. kPa. The parts add up to the whole, as Dalton's law says.
What if? A climber high on a mountain breathes air at a third of this pressure. The oxygen fraction is still , but its partial pressure is a third. That is why breathing is hard up there.
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