Question

the number of molecules of oxygen in 1 litre of air

Answer 1

1 litre of air contains 80% OO22 by volume. So OO22is 0.8 L.

At STP, 1 mole of OO22 occupies 22.4 L. Hence, we can find how many moles occupy 0.8 L by n=0.822.4=0.0357n=0.822.4=0.0357.

1 mole contains an avagadro number(6.023×10236.023×1023) of particles. So, 0.0357 moles contains 0.0357×6.023×1023=2.15×10230.0357×6.023×1023=2.15×1023 molecules