1. Thermo-physical properties 2. Radiation properties - nptel - Indian ...

Mechanical Measurements Prof S.P.Venkatesan
Indian Institute of Technology Madras
Example 5
A pellet of benzoic acid weighing 0.103 g is burnt in a bomb calorimeter and the
temperature of the calorimeter increases by 2.17°C. What is the heat capacity of
the calorimeter?
Consider the combustion reaction. 7.5 moles of oxygen are required for
complete combustion of benzoic acid.
CHO + 7.5O→ 7CO+ 3HO
7 6 2 2 2 2
Solid Gas Gas Liquid
The change in the number of moles during the reaction is given by
Δ n = n2 − n1 = 7
− 7.5
=−0.5
CO2 in O2 in
the product t he reac tan t
Per mole of benzoic acid basis, we have
Δ U=ΔH−ΔnℜT =−3227 −( − 0.5) × 8.3143× 298
=−3225.8
kJ / mol
The molecular weight of benzoic acid is calculated as
Molecular weight = 12.011× 7
+ 1.008× 6
+ 15.9995× 2
C H2
O
= 122.12 g / mol
0.103
0.000843 mol
Thus 0.103 g of benzoic acid corresponds to 122.12 =
. The heat
released by combustion of this is
Q = 0.000843× 3225.8 =
2.7207 kJ