Question

The enthalpy of combustion of $C _{8} H _{6}$ is - $3250\, kJ$ when $0.39 \,g$ of $C _{6} H _{6}$ is burnt in excess of oxygen in an open vessel, the amount of heat evolved is :

• A. 8.32 kJ
• B. 12.36 kJ
• C. 16.25 kJ
• D. 20.74 kJ

Answer 1

Answer: (C)

Combustion of benzene can be represented as follows
$C _{6} H _{6}+ O _{2} \longrightarrow 6 CO _{2}+ H _{2} O +3250 kJ$
$78\, g$ of $C _{6} H _{6}$ produce $3250\, kJ$ heat
$18 $ will produce $\frac{3250}{78} \,kJ$ heat
$39 \,g$ will produce $=\frac{3250}{78} \times 0.39 \,kJ$
$=16.25 kJ$ heat is liberated
Enthalpy of combustion is the amount of heat liberated when one mole of a substance is burnt in excess of air.