The volume of oxygen gas required at STP is necessary for the complete combustion of 10L methane measured at 30$°$C and 760 mm of Hg pressure is

${\mathrm{P}}_1=760\mathrm{mm},{\mathrm{P}}_2=760\mathrm{mm}$

${\mathrm{V}}_1=10\mathrm{lit}, {\mathrm{V}}_2=?$

${\mathrm{T}}_1=30°\mathrm{C}=303\mathrm{K}, {\mathrm{T}}_2=273\mathrm{K}$

P=Constant

$\frac{{\mathrm{P}}_1{\mathrm{V}}_1}{{\mathrm{T}}_1}=\frac{P_2{V}_2}{T_2}$

$\Rightarrow \frac{{\mathrm{V}}_1}{{\mathrm{T}}_1}=\frac{V_2}{T_2}$

$\Rightarrow \frac{10}{303}\times 273={\mathrm{V}}_2$

$\mathrm{V}2=9.009\mathrm{lit}$

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

$1 \mathrm{mole} {\mathrm{CH}}_4\to 2 \mathrm{mole} \mathrm{of} {\mathrm{O}}_2$

9.009 lit –––– ?

$=2\times 9.009$

=18.01 lit .