Question

The heat liberated when 1.89 g of benzoic acid is burnt in a bomb calorimeter at 25°C and it increases the temperature of 18.94 kg of water by 0.632°C. If the specific heat of water at 25°C is 0.998 cal/g-deg, the value of the heat of combustion of benzoic acid is

(a) 881.1 kcal

(b) 771.124 kcal

(c) 981.1 kcal

(d) 871.2 kcal

Answer 1

Correct option  (b) 771.124 kcal

Explanation:

Given: Weight of benzoic acid= 1.89 g;

Temperature of bomb calorimeter = 25°C =298 K;

Mass of water (m)=18.94 kg= 18940 g; Increase in temperature (ΔT) = 0.632°C and specific heat of water (s) = 0.998 cal/g-deg.

We know that, heat gained by water or heat liberated by benzoic acid (Q)=m sΔT

Since 1.89 g of acid liberates 11946.14 cal of heat, 

therefore heat liberated by 122 g of acid

(where 122 g is the molecular weight of benzoic acid)