Question

The complete combustion of salicylic acid releases 21.90 kJ of energy per gram of salicylic acid. In a particular bomb calorimeter (initially at room temperature), the combustion of 0.2476 g of salicylic acid, in the presence of excess oxygen, causes the temperature of the calorimeter to rise by 2.74 °C. When a 0.3522-g sample of an unknown organic substance is similarly burned in the same calorimeter, the temperature rises by 3.88 °C. What is the energy of combustion per unit mass of the unknown substance?

Answer 1

First, determine the calorimeter constant using the known substance.
The calorimeter constant incorporates the specific heat of the calorimeter fluid (likely water) and its mass.

Heat = 21.90 kJ/g * 0.2476 g = 5.4224 kJ

5.4224 kJ = c∆T
5.4224 kJ = c(2.74°C)

c = 1.979 kJ/°C

Now, we use the same calorimeter constant on our unknown substance.

Heat = 1.979 kJ/°C * 3.88°C = 7.678 kJ

Now, find the heat of combustion per gram:

7.678 kJ / 0.3522 g = 21.80 kJ/g