Question

How many oxygen gas molecules do you need to make 25 g of water vapor (H2O)? Balanced chemical equation:

Answer 1

H₂O is formed from the elements H₂ and O₂ following the shown chemical equation:

$2H_2(g)+O_2(g)\rightarrow 2H_2O(g)$

So, the moles of O₂ is equal to half of moles of H₂O produced.

Molar mass of H₂O = 18 g.mol^-1

Thus, 25 g water = (25 / 18) mol = 1.389 mol.

Thus, moles of O₂ needed = (0.5 x 1.389) mol = 0.6945 mol

Now, 1 mol of O₂ is equal to 6.023 x 10²³ molecules of O₂.

Thus, no of molecules of O₂ = (0.6945 x 6.023 x 10²³) = 4.183 x 10²³.