The minimum number of moles of O 2 required for complete combustion of 1 mole of propane and 2 moles of…

(1) Combustion of propane.

$$\begin{gathered} {\mathrm{C}}_3{\mathrm{H}}_8 + 5{\mathrm{O}}_2\left(\mathrm{g}\right)\to 3{\mathrm{CO}}_2\left(\mathrm{g}\right)+4{\mathrm{H}}_2\mathrm{O} \\ 1 \mathrm{mole} 5 \mathrm{mole} \end{gathered}$$

(2) Combustion of butane

$$\begin{gathered} {\mathrm{C}}_4{\mathrm{H}}_{10}+\frac{13}{2}{\mathrm{O}}_2\to 4{\mathrm{CO}}_2\left(\mathrm{g}\right)+5{\mathrm{H}}_2\mathrm{O} \\ 1 \mathrm{mole} \frac{13}{2} \mathrm{mole} \end{gathered}$$

so total mole of ${\mathrm{O}}_2$ $=5+13=18$