' $x$ ' grams of calcium carbonate was completely burnt in air. The weight of the solid residue formed is $28 g$. What is the value of ' $x$ ' (in grams)?

 The balanced chemical reaction is as follows:

${CaCO}_3\to CaO+{CO}_2( g)$

From the above reaction we can state that $1 \mathrm{mol} \mathrm{CaO}=1 \mathrm{mol} \mathrm{of} {\mathrm{CaCO}}_3$

Therefore, Mass of ${CaCO}_3$ that produces $28 g$ of $CaO$

$=\frac{(28\times 100)}{56}=50 g$

Hence the correct option is (D).