State the formula to calculate the percentage purity of pure substances.

In a reaction, $5.0 \mathrm{g}$ of calcium carbonate on heating gave $2.8 \mathrm{g}$ of calcium oxide and $2.2 \mathrm{g}$ of carbon dioxide. Show that these observations are in agreement with the law of conservation of mass.

 $\mathrm{Calcium} \mathrm{carbonate} \stackrel{\mathrm{heat}}{\to } \mathrm{Calcium} \mathrm{oxide} + \mathrm{Carbon} \mathrm{dioxide}$

Select from the following figure(s) that correctly represent(s) the experimental set-up for the verification of conservation of mass in a chemical reaction.
${\mathrm{BaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{BaSO}}_4\left(\mathrm{s}\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$