What number should be substituted for the question mark to balance the equation and why is that the correct number? 2c4h10 +? o2 → 8co2 + 10h2o + heat a. the number 8 is needed to balance the equation. the right side of the equation shows 16 oxygen atoms, so the left side needs 8 × o2. b. the number 9 is needed to balance the equation. the right side shows 8 oxygen atoms in carbon dioxide and 10 oxygen atoms in water for a total of 18 oxygen atoms. c. the number 13 is needed to balance the equation. the right side of the equation shows 26 oxygen atoms, so the left side needs 13 × o2. d. the number 5 is needed to balance the equation. the right side shows 10 oxygen atoms, so the left side needs 5 oxygen atoms.