Stoichiometry: 4.0 g of H2 reacts completely with O2 to form H2O. What mass of H2O is produced? Give the final numeric answer in grams as the headline.

2H₂(g) + O₂(g) → 2H₂O(l)

Molar mass of H₂ = 2 × 1.008 g/mol = 2.016 g/mol

Moles of H₂ = Mass / Molar mass
Moles of H₂ = 4.0 g / 2.016 g/mol ≈ 1.984 mol H₂

From the balanced equation, 2 moles of H₂ produce 2 moles of H₂O. This is a 1:1 mole ratio.
Moles of H₂O = Moles of H₂ ≈ 1.984 mol H₂O

Molar mass of H₂O = (2 × 1.008 g/mol) + (1 × 15.999 g/mol) = 2.016 g/mol + 15.999 g/mol = 18.015 g/mol

Mass of H₂O = Moles × Molar mass
Mass of H₂O = 1.984 mol × 18.015 g/mol ≈ 35.74 g

The given mass (4.0 g) has two significant figures, so the final answer should also be rounded to two significant figures.
Mass of H₂O ≈ 36 g