Question

How much energy is released if 3705g of octane are burned

Answer 1

Chemical reaction: C₈H₁₈ + 25/2O₂ → 8CO₂ + 9H₂O; ΔH = -5500 kJ/mol.
m(C₈H₁₈) = 3705 g.
n(C₈H₁₈) = m(C₈H₁₈) ÷ M(C₈H₁₈).
n(C₈H₁₈) = 3705 g ÷ 114 g/mol.
n(C₈H₁₈) = 32,5 mol.
Make proportion: 1 mol(C₈H₁₈) : 5500 kJ = 32,5 mol : Q.
Q = 178750 J ÷ 1000 J/kJ = 178,75 kJ of energy is released  when octane is burned.