Write the general formula for a combustion reaction of a hydrocarbon CxHy with O2 and the products.

• A. CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O
• B. CxHy + (x + y/4) O2 → x CO + (y/2) H2O
• C. CxHy + O2 → CO2 + H2O
• D. CxHy + (x/2) O2 → x CO2 + (y/4) H2O

Answer: (A)

Balancing complete combustion of a hydrocarbon involves using oxygen to form carbon dioxide and water. Each carbon in the hydrocarbon becomes one CO2, so you get x molecules of CO2. The hydrocarbon has y hydrogens, so it produces y/2 molecules of H2O. To balance oxygen, count atoms on both sides: the CO2 contributes 2x oxygens, and the water contributes y/2 oxygens, totaling 2x + y/2 O atoms on the product side. Each O2 brings 2 oxygens, so you need (2x + y/2)/2 = x + y/4 O2 on the reactant side. This yields the balanced equation: CxHy + (x + y/4) O2 → x CO2 + (y/2) H2O. The given form matches this balance exactly. The other forms fail because they would misbalance oxygen or carbon (for example, producing CO instead of CO2, or using an O2 coefficient that does not balance all atoms).