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Refer to: https://en.wikipedia.org/wiki/Glutaric_acid
Molar mass of C₅H₈O₄ = 132.12 g/mol

No. of moles of C₅H₈O₄ = (1.0930 g) / (132.12 g/mol) = 8.2728 × 10⁻³ mol

Heat released in the reaction
= (Heat absorbed by the water) + (Heat absorbed by the calorimeter)
= [(1.361 × 10³) × 4.184 × (27.12 - 24.63)] + [858.7 × (27.12 - 24.63)] J
= 16317 J
= 16.317 kJ

As heat is given off in the combustion, the heat of combustion is negative.
The heat of combustion of C₅H₈O₄
= -(16.317 kJ) / (8.2728 × 10⁻³ mol)
= -1972 kJ/mol