The difference between $\mathrm{\Delta H}\text{ and }\mathrm{\Delta U}(\mathrm{\Delta H}-\mathrm{\Delta U})$when the combustion of one mole of heptanes $\left(\mathrm{\ell }\right)$ is carried out at a temperature “T” is equal to

$$\begin{gathered} \begin{array}{l}{\mathrm{C}}_7{\mathrm{H}}_{16}+11{\mathrm{O}}_2\stackrel{\mathrm{\Delta }}{⟶}7{\mathrm{CO}}_2+8{\mathrm{H}}_2\mathrm{O}\\ \left(\mathrm{\ell }\right)\left(\mathrm{g}\right) \left(\mathrm{g}\right) \left(\mathrm{\ell }\right)\\ \mathrm{\Delta H}-\mathrm{\Delta U}=\mathrm{\Delta nRT} \\ \mathrm{\Delta n}[\mathrm{Number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{gaseous} \mathrm{products} – \mathrm{Number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{gaseous} \mathrm{reactents}] \\ \begin{array}{l}\mathrm{\Delta n}=7-11=-4\\ \therefore \mathrm{\Delta H}-\mathrm{\Delta U}=-4\mathrm{RT}\end{array}\end{array} \end{gathered}$$