How much oxygen is needed to form 50 grams of water?

Answer 1

2H2 + O2 --> 2H2O
mols H2O = grams/molar mass = ?
Using the coefficients in the balanced equation, convert mols H2O to mols O2.
If you want amount O2 in grams, then g O2 = mols O2 x molar mass O2.