The minimum number of moles of O₂ required for complete combustion of 1 mole of propane and 2 moles of butane is

(i) Combustion of propane :

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

(ii) Combustion of butane :

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

$1 \mathrm{mole}\Rightarrow \frac{13}{2}\mathrm{mole}$

$2 \mathrm{mole} = \frac{13}{2}\times 2=13$

Total moles of O₂ = 13 + 5 = 18 mole