The molar enthalpy of combustion of graphite is -400 kJ mol ⁻¹ . A certain mass of graphite is burnt in a bomb calorimeter of heat capacity 50 kJ K ⁻¹ . If the temperature of the calorimeter rises by 4 K , the mass of graphite burnt is _____ g. (Assume graphite to be pure carbon)

Step-by-step solution

The combustion reaction of graphite is: C(s) + O₂(g) CO₂(g) For this reaction, the change in gaseous moles n_g = 1 - 1 = 0 . Therefore, the molar internal energy of combustion is equal to the molar enthalpy of combustion: U = H = -400 kJ mol ⁻¹ . The total heat absorbed by the calorimeter is: q = C T = 50 4 = 200 kJ . Since 400 kJ of heat is released by 1 mole of graphite, the number of moles of graphite burnt to release 200 kJ is: n = 200 400 = 0.5 mol . Mass of graphite burnt = n Molar mass of carbon = 0.5 12 = 6