\(Đặt:n_{CH_4}=a\left(mol\right);n_{C_2H_4}=b\left(mol\right)\left(a,b>0\right)\\ PTHH:CH_4+2O_2\rightarrow\left(t^o\right)CO_2+2H_2O\\ C_2H_4+3O_2\rightarrow\left(t^o\right)2CO_2+2H_2O\\ \Rightarrow\left\{{}\begin{matrix}a+b=0,25\\2a+3b=0,625\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=0,125\\b=0,125\end{matrix}\right.\\ a,m_{hh}=m_{CH_4}+m_{C_2H_4}=16.0,125+28.0,125=5,5\left(g\right)\\ b,V_{CO_2\left(đktc\right)}=22,4.\left(a+2b\right)=8,4\left(l\right)\)