1 mole of H₂ reacts with 0.5 mole of O₂ to form 1 mole of H₂O.
At STP, the volume is directly proportional to moles.
Since 1 mole of H₂ reacts with 0.5 mole of O₂, 20 ml of H₂ reacts with 10ml of O₂ and form 10ml of H₂O.
volume of 1 mole H₂O = 22.4 l = 22400 ml
number of moles in 10ml H₂O =
number of molecules in 1 mole =
number of molecules of water in 10ml =
So 2.688×10²° molecules of water are produced.