a,
$n_{CH_3COONa}=\dfrac{12,3}{82}=0,15(mol)$
$CH_3COONa+NaOH\buildrel{{CaO, t^o}}\over\longrightarrow CH_4+Na_2CO_3$
$\Rightarrow n_{Na_2CO_3}=n_{CH_4}=0,15(mol)$
$T=0+273=273K, P=1atm\Rightarrow$ đktc
$V_{CH_4}=0,15.22,4=3,36l$
b,
$CaO+2HCl\to CaCl_2+H_2O$
$Na_2CO_3+2HCl\to 2NaCl+CO_2+H_2O$
$\Rightarrow n_{CO_2}=0,15(mol)$
$\dfrac{n_{KOH}}{n_{CO_2}}=1,33$
$\Rightarrow$ tạo $KHCO_3$ (x mol), $K_2CO_3$ (y mol)
$KOH+CO_2\to KHCO_3$
$2KOH+CO_2\to K_2CO_3+H_2O$
$\Rightarrow x+y=0,15; x+2y=0,2$
$\Rightarrow x=0,1; y=0,05$
$\to m_{\text{muối}}=0,1.100+0,05.138=16,9g$