What mass of oxygen is needed for the complete combustion of 4.00×10−3 g of methane?

Molar mass of CH₄ (methane) = (12.0 + 1.0×4) g/mol = 16.0 g/mol
Molar mass of O₂ (oxygen) = 16.0 × 2 g/mol = 32.0 g/mol

CH₄ + 2O₂ → CO₂ + 2H₂O
Mole ratio CH₄ : O₂ = 1 : 2

No. of moles of CH₄ combusted = (4.00 × 10⁻³ g) / (16.0 g/mol) = 2.50 × 10⁻⁴ mol
No. of moles of O₂ needed = (2.50 × 10⁻⁴ mol) × 2 = 5.00 × 10⁻⁴ mol
Mass of O₂ needed = (5.00 × 10⁻⁴ mol) × (32.0 g/mol) = 1.6 × 10⁻² g