The minimum number of moles of

$\underset{\text { 1mole }}{\mathrm{C}_{3} \mathrm{H}_{8}}+\underset{\text { 5mole }}{\mathrm{SO}_{2}} \rightarrow 3 \mathrm{Co}_{2}+4 \mathrm{H}_{2} \mathrm{O}$

For 1 mole propane combustion 5 mole $\mathrm{O}_{2}$ required

$\mathrm{C}_{4} \mathrm{H}_{10}+\frac{13}{2} \mathrm{O}_{2} \rightarrow 4 \mathrm{Co}_{2}+5 \mathrm{H}_{2} \mathrm{O}$

$\begin{array}{ll}1 \text { mole } & 6.5 \text { mole } \\ 2 \text { mole } & 13 \text { mole }\end{array}$

For 2 moles of butane 13 mole of $\mathrm{O}_{2}$ is required total moles $=13+5=18$