Question

High-purity benzoic acid (C6H5COOH; delta Hrxn for combustion = -3227 kJ/mol) is used as a standard for calibrating bomb calorimeters. A 1.221 -g sample burns in a calorimeter (heat capacity = 1365 J/Degrees celsius) that contains exactly 1.940 kg of water. What temperature change is observed?

Answer 1

First calculate the number of moles of benzoic acid

= 1.221 g/ 122.12 g / mole

= 0.01 mole

The total heat of benzoic acid

= number of moles * delta Hrxn for combustion

= 0.01 mole*-3227 kJ/mol

= - 32.27 KJ

This amount of heat is absorbed by water to raise the temperature of water;

heat release by benzoic acid = heat absorbed by water

=>   32.27 KJ = Q = m*C*change in T

For the Water:
m= =1.940 kg = 1940 g

c, specific heat ; 4.18 J/g.C

therefore;

32.27 KJ *1000 J/ 1 kj= Q = 1940 g *4.18 J/g.C *change in T

change in T = 3.98 c