chemistry question

Question

For a particular isomer of  the combustion reaction produces  of heat per mole of  consumed, under standard conditions.

What is the standard enthalpy of formation of this isomer of

ΔH∘f = _____ kJ/mol

Answer 1

Use the formula ∆Hrxn = 5099.5 kJ/mol = [8x∆HºCO₂(g) + 9x∆HºH₂O(g)] - ∆HºC8H18(g) + 25x∆HºO₂(g)

Look up values in the table for everything except C8H18(g) and solve for ∆HºC8H18