\(\left\{{}\begin{matrix}n_{CH_4}+n_{H_2}=\dfrac{6,72}{22,4}=0,3\\\dfrac{2.n_{H_2}}{16.n_{CH_4}+2.n_{H_2}}.100\%=30\%\end{matrix}\right.\)
=> \(\left\{{}\begin{matrix}n_{CH_4}=\dfrac{21}{310}\left(mol\right)\\n_{H_2}=\dfrac{36}{155}\left(mol\right)\end{matrix}\right.\)
PTHH: CH4 + 2O2 --to--> CO2 + 2H2O
\(\dfrac{21}{310}\)---------------------->\(\dfrac{21}{155}\)
2H2 + O2 --to--> 2H2O
\(\dfrac{36}{155}\)------------>\(\dfrac{36}{155}\)
=> \(m_{H_2O}=\left(\dfrac{21}{155}+\dfrac{36}{155}\right).18=\dfrac{1026}{155}\left(g\right)\)