# What coefficient would the O_2 have after balancing C_4H_8 + O_2 -> CO_2 + H_2O?

The balancement of the reaction requires the conservation of mass, each atom of carbon in ${C}_{4} {H}_{8}$ consumes one ${O}_{2}$ molecule, and every two atoms of hydrogen one oxygen atom is consumed.
So 4 molecules of ${O}_{2}$ used by carbon add to 2 molecules of ${O}_{2}$ used by hydrogen, in total 6 molecules of ${O}_{2}$
${C}_{4} {H}_{8} + 6 {O}_{2} \rightarrow 4 C {O}_{2} + 4 {H}_{2} O$