In the reaction $2H_2 + O_2 \rightarrow 2H_2O$, what mass of water is produced when 4 g of hydrogen reacts completely with excess oxygen? [H = 1, O = 16]

Moles of H₂ = 4/2 = 2 mol. From the equation, 2 mol H₂ produces 2 mol H₂O. Molar mass of H₂O = 18 g/mol, so mass of H₂O = 2 × 18 = 36 g. Option A (18 g) corresponds to only 1 mol of water, option C (32 g) is the mass of oxygen consumed, and option D (9 g) is half a mole of water.