$ 2M_xO_y+ 2yCO \rightarrow 2xM+ 2yCO_2$
$ 2M+ 2nHCl \rightarrow 2MCln+ nH_2$
=> nCO= nCO2= $\frac{13,44}{22,4}$= 0,6 mol
=> mCO= 16,8g; mCO2= 26,4g
BTKL, mM= 34,8+16,8-26,4= 25,2g
nH2= 0,45 mol => nM= $\frac{0,9}{n}$ mol
=> MM= $\frac{25,2n}{0,9}$= 28n
n=2 => MM= 56. Vậy M là Fe
Mặt khác, mO= 34,8-25,2= 9,6g
=> nO= 0,6 mol
nFe= $\frac{0,9}{2}$= 0,45 mol
=> nFe: nO= 0,45: 0,6= 3:4
Vậy oxit sắt là $Fe_3O_4$