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

1g of salicylic acid releases upon complete combustion 21.90 kJ of enegy.

0.2476 g of salicylic acid releases upon complete combustion will be 5.42 kJ of enegy. (that is 0.2476*21.90)

We know that

Energy = (Mass) ( Sp. Heat Capacity ) (Change in Temperature).

Energy = 5.42 kJ = 5.42 10³ J

mass = 0.2476 g = 0.2476* 10^-3 Kg

Change in Temperature = 2.74 °C or 2.74 K

Sp. Heat Capacity = C ?

5.42 *10³ = 0.2476 10^-3 *C* 2.74

C = 5.42*10³ /0.676* 10^-3= 8 *10⁶ J/kg/K

For unknown:

E = m * C * (delta)T

m = 0.3522 g = 0.3522 * 10^-3 kg

C =8 *10⁶ J/kg/K ( the same calorimeter was used for the unknown compound, so specific heat capacity will be the same)

(delta)T = 3.88 °C = 3.88 K

E = (0.3522 * 10^-3)(8 *10⁶)(3.88)

E = 10.9 * 10³ J

or

E = 10.9 KJ

Energy of combustion (per unit mass) = Amount of Energy / Mass of unknown

= 10.9 kJ/(0.3522 * 10^-3 kg) = 31* 10³