Answer is: mass oxygen is 129,28 grams.
Chemical reaction: 2C₁₈H₃₈ + 55O₂ → 36CO₂ + 38H₂O.
m(C₁₈H₃₈) = 37,5 g.
n(C₁₈H₃₈) = m(C₁₈H₃₈) ÷ M(C₁₈H₃₈).
n(C₁₈H₃₈) = 37,5 g ÷ 254 g/mol.
n(C₁₈H₃₈) = 0,147 mol.
From chemical reaction: n(C₁₈H₃₈) : n(O₂) = 2 : 55.
n(O₂) = 4,04 mol.
m(O₂) = 4,04 mol · 32 g/mol.
m(O₂) = 129,28 g.
