Question

. A 0.500 g sample of naphthalene (C10H8) is burned in a bomb calorimeter containing 650 grams of water at an initial temperature of 20.00 oC. After the reaction, the final temperature of the water is 26.4ºC. The heat capacity of the calorimeter is 420 J/oC. Using these data, calculate the heat of combustion of naphthalene in kJ/mol.

Answer 1

Sol.

As Mass of naphthalene , C10H8 = 0.500 g

Molar Mass of C10H8 = 128.1705 g/mol

So , Moles of C10H8 = n

= 0.500 / 128.1705 = 0.0039 mol

Now ,  

Mass of water = m = 650 g

Change in temperature = deltaT

= Final temperature - Initial temperature

= 26.4 - 20.00  

= 6.4 °C

Specific Heat capacity of calorimeter = C = 420 J/°C

Specific Heat capacity of water = Cw = 4.184 J / g°C   

So ,  

Heat of combustion of naphthalene  

= - ( ( m × Cw × deltaT ) + ( C × deltaT ) ) / n

= - ( ( 650 × 4.184 × 6.4 ) + ( 420 × 6.4 ) ) / 0.0039

= - 20093.44 / 0.0039

= - 5152164.102 J / mol

=   - 5152.164 KJ / mol