Volume of CO₂ obtained at STP by the complete decomposition of 8.4 g of MgCO₃ (90% pure) at STP will be

$$\begin{gathered} \begin{array}{cc}{\mathrm{MgCO}}_3\left(\mathrm{s}\right)\stackrel{\mathrm{\Delta }}{⟶}\mathrm{MgO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ 84\mathrm{g} 1 \mathrm{mole}& \end{array} \end{gathered}$$
Volume of CO₂ obtain at STP
$$\begin{gathered} =\frac{8.4}{84}\times \frac{90}{100}\times 22.4 \mathrm{L} \\ =2.016\mathrm{L}\simeq 2.02\mathrm{L} \end{gathered}$$