please explain how to do it I have no idea where to start! Thank You

If you burn 30.2 g of hydrogen and produce 2.70 × 102 g of water, how much oxygen reacted?

1. Write and balance the equation.

2H2 + O2 ==> 2H2O

2. Convert 270 g H2O to mols.
mols = grams/molar mass = ?

3. Using the coefficients in the balanced equation, convert mols H2O to mols of what you want (in this case oxygen). You can see from the equation that 2 mopls H2O needs 1 mol O2.

4. If you want to convert to grams O2, then grams O2 = mols O2 x molar mass O2.

Post your work if you get stuck.