In the combustion of hydrogen gas, hydrogen reacts with oxygen from the air to form water vapor.
hydrogen+oxygen-> water
if you burn 39.7 g of hydrogen and produce 355g of water, how much oxygen reacted?

The reaction equation:
2H₂ + O₂ → 2H₂O
Moles = mass/molecular mass
The moles of Hydrogen present:
39.7/2 = 19.85 moles
The moles of water present:
355/18 = 19.72
The moles of water that should have been formed according to equation:
moles Hydrogen = Moles water = 19.85
Because less than the predicted amount of water is formed, we know that Hydrogen is in excess and Oxygen is the limiting reactant. We will calculate the moles of Oxygen using moles of water.
Molar ratio of Oxygen : Water = 1:2
Moles of Oxygen = 19.72/2 = 9.86
Mass of Oxygen = 315.52 grams