Question

f you burn 28.2 g of hydrogen and produce 252 g of water, how much oxygen reacted?

Answer 1

2 H2 + O2 = 2 H2O
The product is a better measure of how much O2 actually reacted, so:
(252 g H2O) / (18.01532 g H2O/mol) x (1 mol O2 / 2 mol H2O) x
(31.99886 g O2/mol) = 224 g O2