Estimate how many molecules you breathe in with a 1.0-L breath of air. Assume at STP, 1 mole gas has a volume of `22.4\ L`. `n = (1.0\ L)/(22.4\ L//mol) = 0.045\ mol` Then `1.0\ L` of air contains `0.045\ mol times 6.02 times 10^23 //mol ≈ 3 times 1^22 ` molecules.