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.