In the reaction C5H8 + O2 ➡ 8H2O + HCO2, what coefficient should be placed in front of O2 to balance the reaction? A. 5
B. 8
C. 9
D. No coefficient is needed

Equalization of chemical reactions can be done using variables. Steps in equalizing the reaction equation:  

•1. gives a coefficient on substances involved in the equation of reaction such as a, b, or c, etc.  

•2. make an equation based on the similarity of the number of atoms where the number of atoms = coefficient × index (subscript) between reactant and product  

•3. Select the coefficient of the substance with the most complex chemical formula equal to 1  

For gas combustion reaction which is a reaction of hydrocarbons with oxygen produces CO₂ and H₂O (water vapor). can use steps:  

Balancing C, H, and the last O  

1. gives  a coefficient

C₅H₈+aO₂ ⇒ 5CO₂ + 8H₂O

O⇒left =2a, right= 5.2+8=18 ⇒ 2a=18⇒a=9

So the coefficient in front of O₂ = 9

But this reaction is not balanced because the number of H is not balanced (8 on the left and 16 on the right, but if we use this equation:

C₅H₈ + 7O₂⇒ 5CO₂ + 4H₂O

then the reaction will be balanced

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