Question

How many grams of water (H_2O}) have the same number of oxygen atoms as 1.0mol of oxygen gas?

Answer 1

1 mole of H2O has Avogadro's number of O atoms (one per molecule).

1 mole of O2 gas has twice Avogadro's number of O atoms (two per molecule)

You need two moles of H2O to have as many O atoms as one mole of O2.