What is the mass of O2(g) consumed in the combustion of 21.5 g cyclohexane, C6H12 to form CO2(g) and H2O(g)?

Molar mass of C₆H₁₂ = (12.0×6 + 1.0×12) g/mol = 84.0 g/mol
Molar mass of O₂ = 16.0×2 = 32.0 g/mol

The balanced equation for the reaction :
C₆H₁₂ + 9O₂ → 6CO₂ + 6H₂O
Mole ratio C₆H₁₂ : O₂ = 1 : 9

No. of moles of C₆H₁₂ consumed = (21.5 g) / (84.0 g/mol) = 0.256 mol
No. of moles of O₂ consumed = (0.256 mol) × 9 = 2.304 mol
Mass of O₂ consumed = (2.304 mol) × (32.0 g/mol) = 73.7 g