How many atoms of Hydrogen are in 67.2 L of H2 at STP?
One mole of any gas at STP has a volume of 22.4 L. So first determine the number of moles of gas you have.
67.2/22.4 = 3 moles of H2
One mole of any substance contains Avogadro's number of atoms = 6.022 x 10^23 atoms.
So multiply number of moles x number of atoms/mole = 1.8066 x 10^24 atoms of H2.