What mass of octane must be burned in order to liberate 5270 kj of heat? δhcomb = - 5471 kj/mol?

As,

5471 kJ heat is given by = 1 mole of Octane

Then,

5310 kJ heat will be given by = X moles of Octane

Solving for X,

X = (5310 kJ * 1 mol) : 5471 kJ

X = 0.970 moles of Ocatne

So, 0.970 moles of Octane will liberate 5310 kJ energy. Now changing moles to mass,

As,

Moles = mass / M. mass

Or,

Mass = Moles * M. mass

Putting values,

Mass = 0.970 mol * 114.23 g/mol

Mass = 110.83 g of Octane