Question

The combustion of 0.1567 g benzoic acid increases the temperature of a bomb calorimeter by 2.57°C. Calculate the heat capacity of this calorimeter. (The energy released by combustion of benzoic acid is 26.42 kJ/g.)A 0.2156-g sample of vanillin (C8H8O3) is then burned in the same calorimeter, and the temperature increases by 3.28°C. What is the energy of combustion per gram of vanillin? Per mole?

Answer 1

Since the energy released by the combustion of benzoic acid is 26.42 KJ/g, therefore the energy released by the combustion of 0.1567 g of benzoic acid will be = 26.42 KJ/g x 0.1567 g = 4.140 KJ

Also this amount of energy increases the temperature of a bomb calorimeter by 2.57⁰C, the heat capacity of calorimeter is will be,C = 4.140 KJ / 2.57⁰ C = 1.61 KJ/⁰C .

Energy of combustion of vanillin which causes 3.28⁰C increase in temperature ---
q = 1.61 KJ/⁰C x 3.28⁰C = 5.28 KJ
This amount of energy released by the combustion of 0.2156 g of vanillin(C₈H₈O₃). Therefore, the energy of combustion per gram of vanillin will be = 5.28 KJ / 0.2156 g = 24.49 KJ/g

Now, the molar mass of vanillin(C₈H₈O₃) will be =(12x 8 + 1 x 8 + 3 x 16)g/mol = 96 + 8 + 48 = 152 g/mol

Required energy of combustion per mole of vanillin is therefore,
24.49 KJ/g x 152 g/mol = 3722.48 KJ/mol