Question

70. When 5.00 g adenine (C5H5N5, 135.1 g/mol) is combusted in a bomb calorimeter, 103 kJ of heat is released and a temperature increase of 23.6 C is noted. In the same calorimeter, 5.00 g guanine (C5H5N5O, 151.1 g/mol) is combusted, and a temperature increase of 19.0 C is observed. What is the enthalpy of combustion per mol of guanine?

Answer 1

Solution :-

103 kJ heat raises the temperature by 23.6 degree Celsius

So lets calculate the amount of heat needed to raise the temperature by 19.0 degree Celsius

19.0 ^oC * 103 kJ / 23.6 ^oC = 82.9 kJ

So this means

5 g guanine gives 82.9 kJ heat

Now lets calculate the heat given by the 1 mole guanine

82.9 kJ * 151.1 g per mol / 5.00 g = 2505 kJ

So 1 mole of guanine can produce 2505 kJ heat

Since the heat is given out so the reaction is exothermic

So the enthalpy of combustion of guanine is -2505 kJ/mol