The chemical formula of water is H2O. One molecule of water has 2 atoms of hydrogen and one atom of oxygen.
1 mole of water requires 1 mole of hydrogen (H2) and 0.5 moles of oxygen (O2).
The molar mass of water is 100/18
This requires 100/18 moles of hydrogen. As the molar mass of hydrogen is 2, it requires 200/18 = 100/9 g of hydrogen.
It requires 0.5*100/18 mole of oxygen. The molar mass of oxygen is 32. The mass of oxygen required is 800/9 g
It requires 100/9 g of hydrogen gas and 800/9 g of oxygen gas to make 100 g of water.