The volume of ${\mathrm{O}}_2$ required for the complete combustion of 120 ${\mathrm{cm}}^3$ of methane gas in a eudiometer tube will be:

${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$

$22400{\mathrm{cm}}^3{\mathrm{CH}}_4\text{ need }{\mathrm{O}}_2$ for complete combustion

$=2\times 22400{\mathrm{cm}}^3$

$120{\mathrm{cm}}^3{\mathrm{CH}}_4\text{ need }{\mathrm{O}}_2$ for complete combustion

$\begin{array}{l}=\frac{2\times 22400{\mathrm{cm}}^3}{22400{\mathrm{cm}}^3}\times 120{\mathrm{cm}}^3\\ =240{\mathrm{cm}}^3\end{array}$