$M_{2}C{O}_3+2HCl\to 2MCl+H_{2}O+C{O}_2$

From reaction,

$\mathrm{Moles\; of} M_{2}C{O}_3=\mathrm{M}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{ }\mathrm{o}\mathrm{f} C{O}_2$

$\frac{\mathrm{Weight} \mathrm{of} {\mathrm{M}}_2{\mathrm{CO}}_3}{\mathrm{Molar} \mathrm{Mass} \mathrm{of} {\mathrm{M}}_2{\mathrm{CO}}_3} =\frac{\mathrm{Weight} \mathrm{of} {\mathrm{CO}}_2}{\mathrm{Molar} \mathrm{mass} \mathrm{of} {\mathrm{CO}}_2}$

$\frac{1}{\mathrm{Molar} \mathrm{Mass} \mathrm{of} {\mathrm{M}}_2{\mathrm{CO}}_3} =\frac{0.01186 \times 44}{44}$

 $\frac{1}{\mathrm{Molar} \mathrm{Mass} \mathrm{of} {\mathrm{M}}_2{\mathrm{CO}}_3}=0.01186$

Molar Mass  $=\frac{1}{0.01186}=84.3$ g/mol