The heat produced on complete combustion of 10 g of a fuel could raise the temperature of 2 kg of water from 20° C to 70° C. Calculate the calorific value of the fuel, if the specific heat capacity of water was 4.2 J/g ° C. Assume that the heat taken by the container is negligible.

Mass of water = 2 kg = 2000 g
Specific heat of water = 4.2 J/g ° C
Rise in temperature, t = 70 ° C – 20 ° C = 50° C
We know that heat produced Q = m × s × t
                                            = 2000 × 4.2 × 50
                                            = 420000 joules
This heat of 420000 joules has been produced by burning 10 g of fuel.
Let us calculate the heat produced by burning 1 g of fuel, hence
10 g of fuel produce heat = 420000 joules
and 1 g of fuel produce heat = = 42000 joule.
Thus, the calorific value of the fuel is 42000 joules per gram.
To convert it into kilojoules per gram, we have to divide by 1000, hence
Calorific value of fuel = kJ/g = 42 kJ/g.