Question

The heat liberated when $1.89\, g$ of benzoic acid is burnt in a bomb calorimeter at $25 ^{\circ} C$ increases the temperature of $18.94\, kg$ of water by $0.632^{\circ} C$. If the specific heat of water at $25^{\circ} C$ is $0.998\, cal/g$ the value of the heat of combustion of benzoic acid is

• A. 881.1 kcal
• B. 771.4 kcal
• C. 981.1 kcal
• D. 871.2 kcal

Answer 1

Answer: (B)

771.4 kcal

Answer 2

Heat capacity, $C_{V} =18.94 \times 0.998 \times 10^{3}$ $=18.902 \times 10^{3} cal / deg$ Moles of benzoic acid $=\frac{1.89}{122}$ $C_{p} =C_{v}+R$ $=18.902 \times 10^{3}+2$ $=18.904 \times 10^{3} cal$ Heat of combustion $=\frac{C_{p} \times \Delta t}{\text { moles }}$ $=\frac{18.904 \times 10^{3} \times 0.632 \times 122}{1.89}$ $=771.2 \times 10^{3} cal$ $=771.2\, kcal$