Câu 20: $B$
$n_{CO_2}=0,4(mol)$
$n_{Ca(OH)_2}=0,35(mol)$
$\dfrac{2n_{Ca(OH)_2}}{n_{CO_2}}=1,75$
$\to$ tạo 2 muối: $CaCO_3$ (x mol), $Ca(HCO_3)_2$ (y mol)
Bảo toàn $Ca$: $x+y=0,35$
Bảo toàn $C$: $x+2y=0,4$
$\to x=0,3; y=0,05$
$\to m_{\downarrow}=100x=30g$
Câu 25: $B$
$n_{H_2O}=\dfrac{12,6}{18}=0,7(mol)$
$n_{Fe}=\dfrac{28}{56}=0,5(mol)$
Đặt $n_{FeO}=x(mol); n_{Fe_2O_3}=y(mol)$
Bảo toàn $O$: $x+3y=0,7$
Bảo toàn $Fe$: $x+2y=0,5$
$\to x=0,1; y=0,2$
$\%m_{FeO}=\dfrac{0,1.72.100}{0,1.72+0,2.160}=18,4\%$
