How many grams of water are produced when 4.0 grams of hydrogen gas (H2) reacts completely with excess oxygen gas (O2)? The balanced equation is 2H2 + O2 -> 2H2O.

Here's the calculation:

1. Molar mass of H2: 2 * 1.008 g/mol = 2.016 g/mol
2. Molar mass of H2O: (2 * 1.008 g/mol) + 15.999 g/mol = 18.015 g/mol
3. Moles of H2: 4.0 g H2 / 2.016 g/mol H2 = 1.984 mol H2
4. Mole ratio from balanced equation (2H2 -> 2H2O): For every 2 moles of H2, 2 moles of H2O are produced, which is a 1:1 ratio.
5. Moles of H2O produced: 1.984 mol H2 * (2 mol H2O / 2 mol H2) = 1.984 mol H2O
6. Mass of H2O produced: 1.984 mol H2O * 18.015 g/mol H2O = 35.74 g H2O

Rounding to two significant figures (based on 4.0 grams H2), the answer is 36 grams of water. If we keep more precision from the molar masses, it's closer to 35.6 grams. Let's use 35.6 g for better precision.