JEE Main 2021 · Chemistry · Chemical Thermodynamics · medium

78 g of benzene (C6H6, M=78 g mol^-1) is burnt completely in excess oxygen in a bomb calorimeter whose heat capacity (including contents) is 10.0 kJ…

Numerical answer

0.78\ \mathrm{g}

of benzene (

\mathrm{C_6H_6}

M=78\ \mathrm{g\,mol^{-1}}

) is burnt completely in excess oxygen in a bomb calorimeter whose heat capacity (including contents) is

10.0\ \mathrm{kJ\,K^{-1}}

. The temperature rises by

3.27\ \mathrm{K}

298\ \mathrm{K}

. The magnitude of the enthalpy of combustion of benzene in

\mathrm{kJ\,mol^{-1}}

, to the nearest integer, is (liquid benzene, products

\mathrm{CO_2}(g)

and

\mathrm{H_2O}(l)

;

R=8.314\ \mathrm{J\,K^{-1}\,mol^{-1}}

)

Show answer and solution

Answer: 3274

Heat released at constant volume:

q_V=10.0\times3.27=32.7\ \mathrm{kJ}

for

0.01

mol, so

\Delta U=-3270\ \mathrm{kJ\,mol^{-1}}

. Reaction:

\mathrm{C_6H_6}(l)+\tfrac{15}{2}\mathrm{O_2}(g)\to6\mathrm{CO_2}(g)+3\mathrm{H_2O}(l)

, so

\Delta n_g=6-7.5=-1.5

. Then

\Delta H=\Delta U+\Delta n_gRT=-3270+(-1.5)(8.314\times10^{-3})(298)=-3270-3.72=-3273.7\ \mathrm{kJ\,mol^{-1}}

. The magnitude to the nearest integer is

3274

Original question written in the JEE Main 2021 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake