Question

You burn 56.0 grams of hydrogen that produces 501 grams of water, how much oxygen reacted?

Answer 1

First write a chemical equation for the process and balance it.

Answer 2

Although @aaronq is entirely correct, I would use conservation of mass first.

Answer 3

50000

Answer 4

That's a much better idea, @JFraser

Answer 5

Supposing the given amounts represent a complete reaction:
Use the Law of Conservation of Mass:
501 g H2O - 56.0 g H2 → 445 g O2