Some Basic Concepts Of Chemistry

At STP ' $x$ ' $g$ of a metal hydrogen carbonate $\left(M \mathrm{HCO}_3\right)$ (molar mass $84 \mathrm{~g} \mathrm{~mol}^{-1}$ ) on heating gives $\mathrm{CO}_2$, which can completely react with 0.2 moles of MOH (molar mass $40 \mathrm{~g} \mathrm{~mol}^{-1}$ ) to give $\mathrm{MHCO}_3$. The value of ' $x^{\prime}$ is