In this question, we have the following reaction:
2 H2 + O2 -> 2 H2O
At STP (standard temperature and pressure), 1 mol of gas is equal to 22.4 Liters of volume, and also equal to 6.022*10^23 molecules or particles, and this is the Avogadro's constant number. For our question, we have 2 moles of H2, 1 mol of O2 and 2 moles of H2O, according to this, we will have:
1.204*10^24 molecules of H2
2 moles of H2
44.8 L of H2
6.022*23 molecules of O2
1 mol of O2
22.4 L of O2
1.204*10^24 molecules of H2O
2 moles of H2O
44.8 L of H2O