if you burn 20.2g of hydrogen and produce 1.80x10^2g of water, how much oxygen reacted?

Answer 1

2H2 + O2 ==> 2H2O
mols H2 = grams/molar mass
Using the coefficients in the balanced equation to convert mols H2 to mols O2.
Now convert mols O2 to what you meant by "how much". If you want grams O2, g O2 = mols O2 x molar mass O2.
If you want volume at STP it is L = mols O2 x 22.4 L/mol = ?