$nX=\frac{21,6}{22,4}=0,96 mol, m anken=20g(ankan k bị hấp thụ) ,$$CH4+ O2\rightarrow CO2 +2H2O\rightarrow nCH4=0,25 mol\rightarrow VCH4=0,25.22,4=5,6l\rightarrow Vanken=21,6-5,6=16l$
$\rightarrow n anken=0,96-0,25=0,71 mol\rightarrow M anken=\frac{20}{0,71}=28\rightarrow C2H4$