The number of moles of oxygen in one litre of air containing $$21\% $$  oxygen by volume, under standard conditions are

Solution :
$$21\% $$  of 1 litre is 0.21 litre.
22.4 litres = 1 $$mole$$  at $$STP$$
$$\therefore $$ 0.21 litre $${\text{ = }}\frac{{0.21}}{{22.4}} = 0.0093\,mol$$