Em tham khảo nha :
\(\begin{array}{l}
{n_{C{O_2}}} = \dfrac{{8,4}}{{22,4}} = 0,375mol\\
{n_C} = {n_{C{O_2}}} = 0,375mol\\
{m_C} = 0,375 \times 12 = 4,5g\\
{n_{{H_2}O}} = \dfrac{{4,5}}{{18}} = 0,25mol\\
{n_H} = 2{n_{{H_2}O}} = 0,5mol\\
{m_H} = 0,5 \times 1 = 0,5g\\
\% C = \dfrac{{4,5}}{5} \times 100\% = 90\% \\
\% H = \dfrac{{0,5}}{5} \times 100\% = 10\%
\end{array}\)