Question

A 1.000 g sample of octane (C8H18) is burned in a bomb calorimeter containing 1200 grams of water at an initial temperature of 25.00ºC. After the reaction, the final temperature of the water is 33.20ºC. The heat capacity of the calorimeter (also known as the “calorimeter constant”) is 837 J/ºC. The specific heat of water is 4.184 J/g ºC. Calculate the heat of combustion of octane in kJ/mol.

Answer 1

moles of C8H18 = mass / molar mass = 1.000 / 114 = 8.77 x 10^-3

dT = T2 - T1 = 33.20 - 25 = 8.2 oC

Q = m Cp dT + Cp dT

Q = 1200 x 4.184 x 8.2 + 837 x 8.2

Q = 48034 J

Q = 48.03 kJ

heat of combustion of octane   = - Q / n

                                                = -48.03 / 8.77 x 10^-3

                                               = - 5477 kJ / mol