CH4的物质的量n=m/M=4/16=0.25mol
CH4+2O2=点燃=CO2+2H2O
1 2
0.25mol n(O2)
n(O2)=0.5mol
标况下O2的体积V=n*Vm=0.5*22.4=11.2L
答：标况下,4g CH4完全燃烧需氧气11.2L.