If you burn 23.4 g of hydrogen and produce 209 g of water, how much oxygen reacted?

The chemical reaction is expressed as:

2H2 + O2 = 2H2O

To determine the amount of oxygen used in the reaction, we use the amount of water produced and the relation of the substances in the reaction we do as follows:

209 g H2O ( 1 mol / 18.02 g ) ( 1 mol O2  / 2 mol H2O ) ( 32 g / 1 mol ) = 185.57 g O2