Question

The combustion of naphthalene (C10H8), which releases 5150.1kj/mol, is often use to calibrate calorimeters. A 1.05-g sample of naphthalene is burned in a calorimeter, producing a temperature rise of 3.86oC. Burning a 1.83-g sample of coal in the same calorimeter causes a temperature change of 4.90oC. What is the energy density of the coal?

Answer 1

Molar mass of naphthalene$=10\times 12+8=128\ g$

Moles in $1.05\ g$ naphthalene$=\frac{1.05}{128}$

Let the total capacity of calorimeter per unit temperature change$=C$

Using energy balance,

$\frac{1.05}{128}\times 5150.5=C\times 3.86\implies C=10.946\ KJ/\degree C$

Now coal is burnt.

Let $S$ be the energy density $($ Energy stored in unit mass $)$ ,

Applying energy balance,

$1.83S=10.946\times 4.90\implies S=29.31\ KJ/g$