18.4g of a mixture CaCO₃ and MgCO₃ on heating gives 4.0g of magnesium oxide.
The volume of CO₂ produced at STP in this process is

${\mathrm{MgCO}}_3\stackrel{∆}{\to }\mathrm{MgO}+{\mathrm{CO}}_2$
84gm              40gm      44gms 

8.4gms           4gms       4.4gms
$4.4\mathrm{gms} \mathrm{of} {\mathrm{CO}}_2=2.24 \mathrm{lit} \mathrm{at} \mathrm{S}.\mathrm{T}.\mathrm{P}$
$$\begin{gathered} {\mathrm{CaCO}}_3\to \mathrm{CaO}+{\mathrm{CO}}_2 \\ 100\mathrm{gm}56\mathrm{gm}44\mathrm{gm} \end{gathered}$$
$$\begin{gathered} 10\mathrm{gms}5.6\mathrm{gm}4.4\mathrm{gm} \\ 4.4\mathrm{gms}{\mathrm{CO}}_2=2.24\mathrm{lit} \end{gathered}$$
$\therefore \text{ volume of }{\mathrm{CO}}_2=2.24+2.24=4.48\mathrm{lit}$