When balancing a combustion equation, an odd number of oxygen atoms on the product side is handled by:

1. Balancing an equation must never change the formulas of the species involved. 2. Seven oxygen atoms on the product side is an odd number, and no integer coefficient works with an oxygen molecule reactant to yield an odd number. 3. So a fractional coefficient is used instead to yield a provisional balanced equation. 4. When balance is achieved, all the equation's coefficients may then be multiplied by a whole number to convert the fractional coefficients to integers. 5. That multiplication does not upset the atom balance. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 4 'Stoichiometry of Chemical Reactions', sections 4.1-4.5_