The octane rating of gasoline is a relationship of the burning efficiency of the given gasoline mixture to the burning efficiency of octane (C8H18). Like most hydrocarbons, octane reacts with oxygen gas to produce carbon dioxide and water. The unbalanced equation for this reaction is

C₈H₁₈(l) + O₂(g) ⇒ CO₂(g) + H₂O(l)

Explanation:

Let's consider the unbalanced equation for the combustion reaction of octane.

C₈H₁₈(l) + O₂(g) ⇒ CO₂(g) + H₂O(l)

We can begin balancing H atoms by multiplying H₂O by 9, and C atoms by multiplying CO₂ by 8.

C₈H₁₈(l) + O₂(g) ⇒ 8 CO₂(g) + 9 H₂O(l)

Finally, we get the balanced equation by multiplying O₂ by 12.5.

C₈H₁₈(l) + 12.5 O₂(g) ⇒ 8 CO₂(g) + 9 H₂O(l)

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.