If oxygen is removed from a sample of air as iron rust, what happens to the particle pressure of oxygen in the air?

Answer : It increases

Rusting is where oxygen binds to iron and forms iron oxide.

So once iron rusts, there is oxygen, just not in air; it's in the iron oxide.

All reactions are reversible, albeit at different rates (the "irreversible" ones are still reversible, but much slower given that they take so much collision luck and energy.

that's what i got you can count the number of atoms