What number should be substituted for the question mark to balance the equation and why is that the correct number? 2C2H6 +?O2 → 4CO2 + 6H2O + heat A. The number 5 is needed to balance the equation. The right side shows 4 oxygen atoms in carbon dioxide and 6 oxygen atoms in water for a total of 10 oxygen atoms. B. The number 3 is needed to balance the equation. The right side shows 6 oxygen atoms, so the left side needs 3 oxygen atoms. C. The number 7 is needed to balance the equation. The right side of the equation shows 14 oxygen atoms, so the left side needs 7 × O2. D. The number 4 is needed to balance the equation. The right side of the equation shows 8 oxygen atoms, so the left side needs 4 × O2.