$a)PTHH:CuO+CO\xrightarrow{t^o}Cu+CO_2\uparrow$

$Fe_2O_3+3CO\xrightarrow{t^o}2Fe+3CO_2$

$\Rightarrow $ X gồm $CO$ dư $x(mol)$ và $CO_2$ $y(mol)$

$\Rightarrow n_{CO(bđ)}=x+y=\dfrac{6,72}{22,4}=0,3(1)$

Mà $M_X=1,175.M_{O_2}=1,175.32=37,6(g/mol)$

$\Rightarrow \dfrac{28x+44y}{x+y}=37,6$

$\Rightarrow 28x+44y=37,6.0,3=11,28(2)$

$(1)(2)\Rightarrow x=0,12(mol);y=0,18(mol)$

$\Rightarrow \%V_{CO_2}=\dfrac{0,18}{0,18+0,12}.100\%=60\%$

$\Rightarrow \%V_{CO(dư)}=100-60=40\%$

$b)m_{CO(p/ứ)}=0,12.28=3,36(g);m_{CO_2}=0,18.44=7,92(g)$

Bảo toàn KL: $m_{oxit}+m_{CO}=m_{KL}+m_{CO_2}$

$\Rightarrow m_{KL}=12+3,36-7,92=7,44(g)$

$\Rightarrow m_O=m_{oxit}-m_{KL}=12-7,44=4,56(g)$