Sol. $$ \underset{1 \mathrm{gm}}{\mathrm{M}_2 \mathrm{CO}_3}+\underset{\text { Excess }}{2 \mathrm{HCl}} \rightarrow \underset{0.02 \mathrm{~mole}}{2 \mathrm{MCl}}+\mathrm{H}_2 \mathrm{O}+\underset{0.01 \text { mole }}{\mathrm{CO}_2} $$ From principle of atomic conservation of carbon atom, Mole of $\mathrm{M}_2 \mathrm{CO}_3 \times 1=$ Mole of $\mathrm{CO}_2 \times 1$ $\frac{1 \mathrm{gm}}{\text { molar mass of } \mathrm{M}_2 \mathrm{CO}_3}=0.01 \times 1$ ∴ Molar mas of $\mathrm{M}_2 \mathrm{CO}_3=100 \mathrm{gm} / \mathrm{mole}$