How many grams of water (H2O) have the same number of oxygen atoms as 7.0 mol of oxygen gas?

Molar mass of H₂O = (1.0×2 + 16.0) g/mol = 18.0 g/mol

(7.0 mol O₂ gas) × (2 mol O atoms / 1 mol O atoms) × (1 mol H₂O / 1 mol O atoms) × (18.0 g H₂O / 1 mol H₂O)
= 252 g H₂O