# When the equation C_2H_4+O_2 -> CO_2+H_2O is balanced using smallest whole numbers, what is the coefficient of the O_2?

${C}_{2} {H}_{4} \left(g\right) + 3 {O}_{2} \left(g\right) \rightarrow 2 C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right)$
The answer is $3$. Is this balanced? For every reactant particle, is there a corresponding product particle? There are $2 C$ reactant particles, and $2 C$ product particles. You have to count up the number of remaining hydrogen and oxygen reactant particles, and see if these correspond to the number of hydrogen and oxygen product particles.