If enthalpy of combustion of carbon to $\mathrm{CO}_2(\mathrm{~g})$ is $-394.0 \mathrm{~kJ}…
If enthalpy of combustion of carbon to $\mathrm{CO}_2(\mathrm{~g})$ is $-394.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$, the enthalpy change for the formation of $17.6 \mathrm{~g}$ of $\mathrm{CO}_2$ from carbon and dioxygen at the same temperature in $\mathrm{kJ}$ is
-157.6
315.2
157.6
-315.2
Solution
Reaction for combusion is
$
\mathrm{C}+\mathrm{O}_2 \longrightarrow \mathrm{CO}_2, \Delta \mathrm{H}=-394 \mathrm{~kJ} \mathrm{~mol}^{-1}
$
$\because$ Molar mass of $\mathrm{CO}_2=12+(2 \times 10)$
and
$
=44 \mathrm{~g}
$
$\because 44 \mathrm{~g}$ of formation of $\mathrm{CO}_2$ gives heat $=-394.0 \mathrm{~kJ}$
$\therefore 17.6 \mathrm{~g}$ of formation of
$
\mathrm{CO}_2 \text {, gives heat }=\frac{-394 \times 17.6}{44}=-157.6 \mathrm{~kJ} \text {. }
$