A $0.800 g$ sample of naphthalene ( $C X_{10} H X_{8}$ , $M = 128 g mo l^{- 1}$ ) is burnt completely in a bomb calorimeter whose heat capacity (including contents) is $10.20 kJ K^{- 1}$ . The temperature rises by $3.25 K$ . The molar internal energy of combustion, $Δ_{c} U$ , is

Answer: A

Heat released

= C Δ T = 10.20 \times 3.25 = 33.15 kJ

for

n = 0.800/128 = 0.00625 mol

. At constant volume

q_{V} = Δ U

, so

Δ_{c} U = - 33.15/0.00625 = - 5304 kJ mo l^{- 1}

. The value

- 33.2

forgets to divide by moles; the last option is

Δ_{c} H

not

Δ_{c} U